Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Calculations for basic and diluted earnings per common share
Net income	$ 89,192	$ 57,983	$ 158,648	$ 114,253
Preferred stock dividends	(2,329)	(2,339)	(4,668)	(4,669)
Net income available to common equity	86,863	55,644	153,980	109,584
Common shareholder dividends	(25,977)	(18,326)	(51,549)	(36,577)
Unvested share-based payment awards	(130)	(110)	(269)	(227)
Undistributed earnings	60,756	37,208	102,162	72,780
Undistributed earnings allocated to common shareholders	60,446	36,980	101,677	72,273
Undistributed earnings allocated to unvested share-based payment awards	310	228	485	507
Undistributed earnings	60,756	37,208	102,162	72,780
Basic
Distributed earnings to common shareholders	25,977	18,326	51,549	36,577
Undistributed earnings allocated to common shareholders	60,446	36,980	101,677	72,273
Total common shareholders earnings, basic	86,423	55,306	153,226	108,850
Diluted
Distributed earnings to common shareholders	25,977	18,326	51,549	36,577
Undistributed earnings allocated to common shareholders	60,446	36,980	101,677	72,273
Total common shareholders earnings, diluted	$ 86,423	$ 55,306	$ 153,226	$ 108,850
Weighted average common shares outstanding (in shares)	170,633	151,573	167,096	151,196
Effect of dilutive common stock awards (in shares)	2,146	1,958	2,080	2,155
Effect of dilutive common stock warrants (in shares)	630	771	744	796
Diluted weighted average common shares outstanding (in shares)	173,409	154,302	169,920	154,147
Basic earnings (loss) per common share (in usd per share)	$ 0.51	$ 0.36	$ 0.92	$ 0.72
Diluted earnings (loss) per common share (in usd per share)	$ 0.50	$ 0.36	$ 0.90	$ 0.71