Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Calculations for basic and diluted earnings per common share
Net income	$ 200,274	$ 188,301	$ 190,509
Preferred stock dividends	(8,903)	(7,155)	(5,002)
Net income available to common equity	191,371	181,146	185,507
Common shareholder dividends	(67,100)	(61,774)	(58,123)
Unvested share-based payment awards	(755)	(626)	(587)
Undistributed earnings	123,516	118,746	126,797
Undistributed earnings allocated to common shareholders	122,205	117,498	125,646
Undistributed earnings allocated to unvested share-based payment awards	1,311	1,248	1,151
Undistributed earnings	123,516	118,746	126,797
Basic
Distributed earnings to common shareholders	67,100	61,774	58,123
Undistributed earnings allocated to common shareholders	122,205	117,498	125,646
Total common shareholders earnings, basic	189,305	179,272	183,769
Diluted
Distributed earnings to common shareholders	67,100	61,774	58,123
Undistributed earnings allocated to common shareholders	122,205	117,498	125,646
Total common shareholders earnings, diluted	$ 189,305	$ 179,272	$ 183,769
Weighted average common shares outstanding (in shares)	148,769	149,350	157,286
Effect of dilutive common stock awards (in shares)	1,192	1,253	968
Diluted weighted average common shares outstanding (in shares)	149,961	150,603	158,254
Basic earnings (loss) per common share (in usd per share)	$ 1.27	$ 1.20	$ 1.17
Diluted earnings (loss) per common share (in usd per share)	$ 1.26	$ 1.19	$ 1.16