Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Calculations for basic and diluted earnings per common share
Net income	$ 350,994	$ 306,771	$ 326,790
Preferred stock dividends	(17,111)	(18,358)	(15,202)
Net income available to common equity	333,883	288,413	311,587
Common shareholder dividends	(115,212)	(111,291)	(111,091)
Unvested share-based payment awards	(849)	(732)	(713)
Undistributed earnings	217,822	176,390	199,784
Undistributed earnings allocated to common shareholders	216,299	175,134	198,424
Undistributed earnings allocated to unvested share-based payment awards	1,523	1,256	1,360
Undistributed earnings	217,822	176,390	199,784
Basic
Distributed earnings to common shareholders	115,212	111,291	111,091
Undistributed earnings allocated to common shareholders	216,299	175,134	198,424
Total common shareholders earnings, basic	331,510	286,425	309,514
Diluted
Distributed earnings to common shareholders	115,212	111,291	111,091
Undistributed earnings allocated to common shareholders	216,299	175,134	198,424
Total common shareholders earnings, diluted	$ 331,510	$ 286,425	$ 309,514
Weighted average common shares outstanding	150,773	153,005	160,534
Effect of dilutive common stock awards	1,214	637	1,398
Diluted weighted average common shares outstanding	151,987	153,642	161,932
Basic earnings per common share	$ 2.20	$ 1.87	$ 1.93
Diluted earnings per common share	$ 2.18	$ 1.86	$ 1.91