Earnings Per Common Share (Computation of Basic and Diluted Earnings per Common Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income	$ 66,559	$ 76,219	$ 112,116	$ 223,642
Distributed and undistributed earnings allocated to participating securities	(2,896)	(3,174)	(4,816)	(9,247)
Income allocated to common stockholders for basic earnings per common share	$ 63,663	$ 73,045	$ 107,300	$ 214,395
Denominator:
Weighted average common shares outstanding	92,405,239	95,075,395	92,918,030	97,113,878
Less average unvested stock awards	(1,183,564)	(1,098,509)	(1,164,317)	(1,147,988)
Weighted average shares for basic earnings per common share	91,221,675	93,976,886	91,753,713	95,965,890
Basic earnings per common share	$ 0.70	$ 0.78	$ 1.17	$ 2.23
Numerator:
Income allocated to common stockholders for basic earnings per common share	$ 63,663	$ 73,045	$ 107,300	$ 214,395
Adjustment for earnings reallocated from participating securities	4	7	3	20
Income used in calculating diluted earnings per common share	$ 63,667	$ 73,052	$ 107,303	$ 214,415
Denominator:
Weighted average shares for basic earnings per common share	91,221,675	93,976,886	91,753,713	95,965,890
Dilutive effect of stock options and certain shared-based awards	171,054	285,934	142,008	303,524
Weighted average shares for diluted earnings per common share	91,392,729	94,262,820	91,895,721	96,269,414
Diluted earnings per common share	$ 0.70	$ 0.77	$ 1.17	$ 2.23