| 
           Debt (Details 2) (USD $) 
        In Thousands, unless otherwise specified  | 
        
           Jun. 30, 2014 
         | 
        
           Dec. 31, 2013 
         | 
      ||||
|---|---|---|---|---|---|---|
| Carrying amount and estimated fair value of debt | ||||||
| Carrying Value | $ 2,099,343 | $ 1,927,703 | ||||
| 
           4.25% Exchangeable Senior Notes 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Carrying Value | 567 | [1] | 563 | [1] | ||
| 
           Carrying Amount 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Variable-rate debt | 607,288 | 657,691 | ||||
| Carrying Value | 2,099,343 | 1,927,703 | ||||
| 
           Carrying Amount | Unsecured senior notes 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Fixed-rate debt | 890,386 | 592,689 | ||||
| 
           Carrying Amount | 4.25% Exchangeable Senior Notes 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Fixed-rate debt | 567 | 563 | ||||
| 
           Carrying Amount | Other fixed-rate debt 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Fixed-rate debt | 601,102 | 676,760 | ||||
| 
           Total Estimated Fair Value 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Variable-rate debt | 609,085 | 657,527 | ||||
| Carrying Value | 2,096,550 | 1,884,473 | ||||
| 
           Total Estimated Fair Value | Unsecured senior notes 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Fixed-rate debt | 897,188 | 575,374 | ||||
| 
           Total Estimated Fair Value | 4.25% Exchangeable Senior Notes 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Fixed-rate debt | 575 | 575 | ||||
| 
           Total Estimated Fair Value | Other fixed-rate debt 
         | 
        ||||||
| Carrying amount and estimated fair value of debt | ||||||
| Fixed-rate debt | $ 589,702 | $ 650,997 | ||||
          
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