Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Calculations for basic and diluted earnings per common share
Net income	$ 85,929	$ 65,001	$ 244,578	$ 179,254
Preferred stock dividends	(2,409)	(2,339)	(7,077)	(7,008)
Net income available to common equity	83,521	62,662	237,501	172,246
Common shareholder dividends	(25,486)	(18,149)	(77,035)	(54,726)
Unvested share-based payment awards	(128)	(105)	(396)	(332)
Undistributed earnings	57,907	44,408	160,070	117,188
Undistributed earnings allocated to common shareholders	57,620	44,150	159,297	116,418
Undistributed earnings allocated to unvested share-based payment awards	288	258	772	770
Undistributed earnings	57,907	44,408	160,070	117,188
Basic
Distributed earnings to common shareholders	25,486	18,149	77,035	54,726
Undistributed earnings allocated to common shareholders	57,620	44,150	159,297	116,418
Total common shareholders earnings, basic	83,106	62,299	236,332	171,144
Diluted
Distributed earnings to common shareholders	25,486	18,149	77,035	54,726
Undistributed earnings allocated to common shareholders	57,620	44,150	159,297	116,418
Total common shareholders earnings, diluted	$ 83,106	$ 62,299	$ 236,332	$ 171,144
Weighted average common shares outstanding	170,516	150,565	168,249	150,983
Effect of dilutive common stock awards	2,188	1,815	2,101	2,081
Effect of dilutive common stock warrants	98	588	526	718
Diluted weighted average common shares outstanding	172,802	152,968	170,876	153,782
Basic earnings per common share	$ 0.49	$ 0.41	$ 1.40	$ 1.13
Diluted earnings per common share	$ 0.48	$ 0.41	$ 1.38	$ 1.11