Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation (Details)

Share-Based Compensation (Details) (USD $)
9 Months Ended 0 Months Ended 9 Months Ended 0 Months Ended
Sep. 30, 2013
Sep. 30, 2012
Mar. 01, 2013
Performance share units
Sep. 30, 2013
Performance share units
Sep. 30, 2013
Restricted shares
Sep. 30, 2013
Mar. 01, 2013
2013 PSU Grants
Performance share units
Potential earned PSUs payout for defined levels of performance under awards              
Earned PSUs payout (as a percent of PSUs granted) on 75th or greater percentile rank       200.00%      
Earned PSUs payout (as a percent of PSUs granted) on 50th percentile rank       100.00%      
Earned PSUs payout (as a percent of PSUs granted) on 25th percentile rank       50.00%      
Performance share units granted on percentile rank below 25th (as a percent)       0.00%      
The number of percentile ranks to fall between to earn interpolated PSUs between such percentile ranks, conditioned on the percentile rank exceeding 25%       2      
Performance period of the award (in years)       3 years      
Assumptions used to value stock awards              
Baseline value per common share (in dollars per share)       $ 25.85      
Expected volatility of common shares (as a percent)       29.50%      
Risk-free interest rate (as a percent)       0.33%      
Stock awards granted (in shares or units)         177,510   69,579
Weighted Average Grant Date Fair Value              
Aggregate grant date fair value     $ 1,900,000   $ 4,700,000    
Weighted average grant date fair value (in dollars per share)       $ 26.84 $ 26.24    
Exercised (in shares) 32,756 44,624       32,756  
Weighted average exercise price (in dollars per share)           $ 19.45  
Aggregate intrinsic value of options exercised (in dollars)           237,000  
Other Share-based Compensation Additional Disclosures              
Shares vested (in shares)         222,381    
Weighted average fair value of shares vested (in dollars per share)         $ 31.54    
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed         $ 5,900,000