Quarterly report pursuant to Section 13 or 15(d)

Share-Based Compensation (Details)

Share-Based Compensation (Details) (USD $)
3 Months Ended 0 Months Ended 3 Months Ended 0 Months Ended
Mar. 31, 2013
Mar. 31, 2012
Mar. 31, 2013
Mar. 31, 2013
Restricted shares
Mar. 01, 2013
Performance share units
Mar. 31, 2013
Performance share units
Mar. 01, 2013
2012 PSU Grants [Member]
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)       117,960     69,579
Weighted Average Grant Date Fair Value              
Aggregate grant date fair value       $ 3,000,000 $ 1,900,000    
Weighted average grant date fair value (in dollars per share)       $ 25.85   $ 26.84  
Weighted average exercise price (in dollars per share)     $ 18.33        
Exercised (in shares) 16,453 5,667 16,453        
Aggregate intrinsic value of options exercised (in dollars)     129,000        
Other Share-based Compensation Additional Disclosures              
Aggregate intrinsic value of restricted common shares, forfeiture restrictions lapsed       $ 4,200,000      
Shares vested (in shares)       161,734      
Weighted average fair value of shares vested (in dollars per share)       $ 33.42