Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Calculations for basic and diluted earnings per common share
Net income	$ 83,339	$ 84,661	$ 86,686	$ 85,929	$ 89,192	$ 69,456	$ 254,686	$ 244,578
Preferred stock dividends	(3,801)			(2,409)			(11,402)	(7,077)
Net income available to common equity	79,539			83,521			243,285	237,501
Common shareholder dividends	(27,091)			(25,486)			(82,741)	(77,035)
Unvested share-based payment awards	(198)			(128)			(506)	(396)
Undistributed earnings	52,250			57,907			160,037	160,070
Undistributed earnings allocated to common shareholders	51,870			57,620			158,970	159,297
Undistributed earnings allocated to unvested share-based payment awards	380			288			1,067	772
Undistributed earnings	52,250			57,907			160,037	160,070
Basic
Distributed earnings to common shareholders	27,091			25,486			82,741	77,035
Undistributed earnings allocated to common shareholders	51,870			57,620			158,970	159,297
Total common shareholders earnings, basic	78,961			83,106			241,711	236,332
Diluted
Distributed earnings to common shareholders	27,091			25,486			82,741	77,035
Undistributed earnings allocated to common shareholders	51,870			57,620			158,970	159,297
Total common shareholders earnings, diluted	$ 78,961			$ 83,106			$ 241,711	$ 236,332
Weighted average common shares outstanding	159,126			170,516			161,727	168,249
Effect of dilutive common stock awards	1,256			2,188			1,334	2,101
Effect of dilutive common stock warrants	0			98			0	526
Diluted weighted average common shares outstanding	160,382			172,802			163,061	170,876
Basic earnings per common share	$ 0.50			$ 0.49			$ 1.49	$ 1.40
Diluted earnings per common share	$ 0.49			$ 0.48			$ 1.48	$ 1.38