Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Calculations for basic and diluted earnings per common share
Net income	$ 188,301	$ 190,509	$ 188,692
Preferred stock dividends	(7,155)	(5,002)	(5,158)
Net income available to common equity	181,146	185,507	183,534
Common shareholder dividends	(61,774)	(58,123)	(54,505)
Unvested share-based payment awards	(626)	(587)	(486)
Undistributed earnings allocated to common shareholders	117,498	125,646	127,592
Undistributed earnings allocated to unvested share-based payment awards	1,248	1,151	951
Undistributed earnings	118,746	126,797	128,543
Basic
Distributed earnings to common shareholders	61,774	58,123	54,505
Undistributed earnings allocated to common shareholders	117,498	125,646	127,592
Total common shareholders earnings, basic	179,272	183,769	182,097
Diluted
Distributed earnings to common shareholders	61,774	58,123	54,505
Undistributed earnings allocated to common shareholders	117,498	125,646	127,592
Total common shareholders earnings, diluted	$ 179,272	$ 183,769	$ 182,097
Weighted average common shares outstanding (in shares)	149,350	157,286	165,584
Effect of dilutive common stock awards (in shares)	1,253	968	218
Diluted weighted average common shares outstanding (in shares)	150,603	158,254	165,802
Basic earnings (loss) per common share (in usd per share)	$ 1.20	$ 1.17	$ 1.10
Diluted earnings (loss) per common share (in usd per share)	$ 1.19	$ 1.16	$ 1.10
Earnings Per Share (Textuals) [Abstract]
Number of antidilutive stock options excluded from the computation of diluted earnings per share (in shares)	1,000	2,000	3,000