Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Calculations for basic and diluted earnings per common share
Net income	$ 65,001	$ 53,816	$ 179,254	$ 145,441
Preferred stock dividends	(2,339)	(2,188)	(7,008)	(6,555)
Net income available to common equity	62,662	51,628	172,246	138,886
Common shareholder dividends	(18,149)	(16,431)	(54,726)	(49,077)
Unvested share-based payment awards	(105)	(177)	(332)	(565)
Undistributed earnings	44,408	35,020	117,188	89,244
Undistributed earnings allocated to common shareholders	44,150	34,645	116,418	88,294
Undistributed earnings allocated to unvested share-based payment awards	258	375	770	950
Undistributed earnings	44,408	35,020	117,188	89,244
Basic
Distributed earnings to common shareholders	18,149	16,431	54,726	49,077
Undistributed earnings allocated to common shareholders	44,150	34,645	116,418	88,294
Total common shareholders earnings, basic	62,299	51,076	171,144	137,371
Diluted
Distributed earnings to common shareholders	18,149	16,431	54,726	49,077
Undistributed earnings allocated to common shareholders	44,150	34,645	116,418	88,294
Total common shareholders earnings, diluted	$ 62,299	$ 51,076	$ 171,144	$ 137,371
Weighted average common shares outstanding (in shares)	150,565	148,708	150,983	148,607
Effect of dilutive common stock awards (in shares)	1,815	1,265	2,081	1,038
Effect of dilutive common stock warrants (in shares)	588	0	718	0
Diluted weighted average common shares outstanding (in shares)	152,968	149,973	153,782	149,645
Basic earnings (loss) per common share (in usd per share)	$ 0.41	$ 0.34	$ 1.13	$ 0.92
Diluted earnings (loss) per common share (in usd per share)	$ 0.41	$ 0.34	$ 1.11	$ 0.92