Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Calculations for basic and diluted earnings per common share
Net income	$ 306,771	$ 326,790	$ 333,562
Preferred stock dividends	(18,358)	(15,202)	(10,784)
Net income available to common equity	288,413	311,587	322,779
Common shareholder dividends	(111,291)	(111,091)	(104,981)
Unvested share-based payment awards	(732)	(713)	(537)
Undistributed earnings	176,390	199,784	217,260
Undistributed earnings allocated to common shareholders	175,134	198,424	216,199
Undistributed earnings allocated to unvested share-based payment awards	1,256	1,360	1,060
Undistributed earnings	176,390	199,784	217,260
Basic
Distributed earnings to common shareholders	111,291	111,091	104,981
Undistributed earnings allocated to common shareholders	175,134	198,424	216,199
Total common shareholders earnings, basic	286,425	309,514	321,181
Diluted
Distributed earnings to common shareholders	111,291	111,091	104,981
Undistributed earnings allocated to common shareholders	175,134	198,424	216,199
Total common shareholders earnings, diluted	$ 286,425	$ 309,514	$ 321,181
Weighted average common shares outstanding	153,005	160,534	167,345
Effect of dilutive common stock awards	637	1,398	1,985
Effect of dilutive common stock warrants	0	0	402
Diluted weighted average common shares outstanding	153,642	161,932	169,732
Basic earnings per common share	$ 1.87	$ 1.93	$ 1.92
Diluted earnings per common share	$ 1.86	$ 1.91	$ 1.89