Operating Leases (Details) $ in Thousands  | 
 Dec. 31, 2017  
USD ($) 
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| Gross minimum future rentals | |
| 2018 | $ 372,420 | 
| 2019 | 329,760 | 
| 2020 | 260,238 | 
| 2021 | 207,727 | 
| 2022 | 175,123 | 
| Thereafter | 484,444 | 
| Total | $ 1,829,712 | 
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- Definition Future minimum rental payments in aggregate as of the balance sheet date under operating leases. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- References No definition available. 
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- Definition Future rental payments receivable within one year of the balance sheet date under an operating lease. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Future rental payments receivable within the fifth year from the balance sheet date under an operating lease. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Future rental payments receivable within the fourth year from the balance sheet date under an operating lease. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Future rental payments receivable within the third year from the balance sheet date under an operating lease. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Future rental payments receivable within the second year from the balance sheet date under an operating lease. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition Future minimum lease payments receivable under operating leases for periods greater than five years following the balance sheet date. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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