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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income	$ 76,508	$ 81,451	$ 45,557	$ 147,423
Distributed and undistributed earnings allocated to participating securities	(3,353)	(3,382)	(1,939)	(6,074)
Income allocated to common stockholders for basic earnings per common share	$ 73,155	$ 78,069	$ 43,618	$ 141,349
Denominator:
Weighted average common shares outstanding	92,409,949	97,451,019	93,177,243	98,150,014
Less average unvested stock awards	(1,207,798)	(1,174,339)	(1,154,589)	(1,173,137)
Weighted average shares for basic earnings per common share	91,202,151	96,276,680	92,022,654	96,976,877
Basic earnings per common share	$ 0.80	$ 0.81	$ 0.47	$ 1.46
Numerator:
Income allocated to common stockholders for basic earnings per common share	$ 73,155	$ 78,069	$ 43,618	$ 141,349
Adjustment for earnings reallocated from participating securities	0	9	0	13
Income used in calculating diluted earnings per common share	$ 73,155	$ 78,078	$ 43,618	$ 141,362
Denominator:
Weighted average shares for basic earnings per common share	91,202,151	96,276,680	92,022,654	96,976,877
Dilutive effect of stock options and certain shared-based awards	705	345,899	126,858	312,821
Weighted average shares for diluted earnings per common share	91,202,856	96,622,579	92,149,512	97,289,698
Diluted earnings per common share	$ 0.80	$ 0.81	$ 0.47	$ 1.45