Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020
Calculations for basic and diluted earnings per common share
Net income	$ 88,809	$ 91,007	$ 94,301	$ 45,214	$ 148,718	$ 45,838	$ 274,117	$ 239,769
Preferred stock dividends	(4,155)			(5,207)			(14,236)	(13,152)
Net income available to common equity	84,655			40,007			259,880	226,618
Common shareholder dividends	(30,323)			(27,676)			(85,604)	(83,607)
Unvested share-based payment awards	(222)			(199)			(634)	(549)
Undistributed earnings	54,109			12,133			173,642	142,462
Undistributed earnings allocated to common shareholders	53,716			12,045			172,436	141,426
Undistributed earnings allocated to unvested share-based payment awards	393			87			1,206	1,036
Undistributed earnings	54,109			12,133			173,642	142,462
Basic
Distributed earnings to common shareholders	30,323			27,676			85,604	83,607
Undistributed earnings allocated to common shareholders	53,716			12,045			172,436	141,426
Total common shareholders earnings, basic	84,039			39,721			258,040	225,032
Diluted
Distributed earnings to common shareholders	30,323			27,676			85,604	83,607
Undistributed earnings allocated to common shareholders	53,716			12,045			172,436	141,426
Total common shareholders earnings, diluted	$ 84,039			$ 39,721			$ 258,040	$ 225,032
Weighted average common shares outstanding	150,046			152,440			151,473	153,175
Effect of dilutive common stock awards	1,096			754			1,228	739
Diluted weighted average common shares outstanding	151,143			153,194			152,701	153,914
Basic earnings per common share	$ 0.56			$ 0.26			$ 1.70	$ 1.47
Diluted earnings per common share	$ 0.56			$ 0.26			$ 1.69	$ 1.46