Earnings Per Common Share (Computation of Basic and Diluted Earnings per Common Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Numerator:
Net income	$ 46,637	$ 48,543	$ 93,094	$ 103,814
Distributed and undistributed earnings allocated to participating securities	(1,810)	(1,934)	(3,582)	(4,086)
Income allocated to common stockholders for basic earnings per common share	$ 44,827	$ 46,609	$ 89,512	$ 99,728
Denominator:
Weighted average common shares outstanding	103,444,183	101,651,265	102,841,376	101,489,190
Less average unvested stock awards	(1,174,496)	(1,205,669)	(1,094,366)	(1,092,262)
Weighted average shares for basic earnings per common share	102,269,687	100,445,596	101,747,010	100,396,928
Basic earnings per common share	$ 0.44	$ 0.46	$ 0.88	$ 0.99
Numerator:
Income allocated to common stockholders for basic earnings per common share	$ 44,827	$ 46,609	$ 89,512	$ 99,728
Adjustment for earnings reallocated from participating securities	5	4	10	9
Income used in calculating diluted earnings per common share	$ 44,832	$ 46,613	$ 89,522	$ 99,737
Denominator:
Weighted average shares for basic earnings per common share	102,269,687	100,445,596	101,747,010	100,396,928
Dilutive effect of stock options	863,380	141,664	763,202	143,066
Weighted average shares for diluted earnings per common share	103,133,067	100,587,260	102,510,212	100,539,994
Diluted earnings per common share	$ 0.43	$ 0.46	$ 0.87	$ 0.99