Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020
Calculations for basic and diluted earnings per common share
Net income	$ 91,007	$ 94,301	$ 148,718	$ 45,838	$ 185,307	$ 194,555
Preferred stock dividends	(4,875)		(4,144)		(10,082)	(7,945)
Net income available to common equity	86,131		144,573		175,226	186,611
Common shareholder dividends	(27,620)		(27,667)		(55,280)	(55,931)
Unvested share-based payment awards	(203)		(222)		(412)	(351)
Undistributed earnings	58,309		116,684		119,533	130,329
Undistributed earnings allocated to common shareholders	57,887		115,750		118,722	129,377
Undistributed earnings allocated to unvested share-based payment awards	422		934		810	953
Undistributed earnings	58,309		116,684		119,533	130,329
Basic
Distributed earnings to common shareholders	27,620		27,667		55,280	55,931
Undistributed earnings allocated to common shareholders	57,887		115,750		118,722	129,377
Total common shareholders earnings, basic	85,506		143,417		174,002	185,307
Diluted
Distributed earnings to common shareholders	27,620		27,667		55,280	55,931
Undistributed earnings allocated to common shareholders	57,887		115,750		118,722	129,377
Total common shareholders earnings, diluted	$ 85,506		$ 143,417		$ 174,002	$ 185,307
Weighted average common shares outstanding	152,042		152,393		152,198	153,547
Effect of dilutive common stock awards	1,339		757		1,275	813
Diluted weighted average common shares outstanding	153,381		153,150		153,473	154,360
Basic earnings per common share	$ 0.56		$ 0.94		$ 1.14	$ 1.21
Diluted earnings per common share	$ 0.56		$ 0.94		$ 1.13	$ 1.20