Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Calculations for basic and diluted earnings per common share
Net income	$ 84,661	$ 86,686	$ 89,192	$ 69,456	$ 171,347	$ 158,648
Preferred stock dividends	(3,801)		(2,329)		(7,601)	(4,668)
Net income available to common equity	80,860		86,863		163,746	153,980
Common shareholder dividends	(27,571)		(25,977)		(55,650)	(51,549)
Unvested share-based payment awards	(205)		(130)		(308)	(269)
Undistributed earnings	53,085		60,756		107,787	102,162
Undistributed earnings allocated to common shareholders	52,690		60,446		107,098	101,677
Undistributed earnings allocated to unvested share-based payment awards	395		310		689	485
Undistributed earnings	53,085		60,756		107,787	102,162
Basic
Distributed earnings to common shareholders	27,571		25,977		55,650	51,549
Undistributed earnings allocated to common shareholders	52,690		60,446		107,098	101,677
Total common shareholders earnings, basic	80,261		86,423		162,748	153,226
Diluted
Distributed earnings to common shareholders	27,571		25,977		55,650	51,549
Undistributed earnings allocated to common shareholders	52,690		60,446		107,098	101,677
Total common shareholders earnings, diluted	$ 80,261		$ 86,423		$ 162,748	$ 153,226
Weighted average common shares outstanding	162,180		170,633		163,049	167,096
Effect of dilutive common stock awards	1,492		2,146		1,469	2,080
Effect of dilutive common stock warrants	0		630		0	744
Diluted weighted average common shares outstanding	163,672		173,409		164,518	169,920
Basic earnings per common share	$ 0.49		$ 0.51		$ 1.00	$ 0.92
Diluted earnings per common share	$ 0.49		$ 0.50		$ 0.99	$ 0.90