Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Calculations for basic and diluted earnings per common share
Net income	$ 148,718	$ 45,838	$ 84,661	$ 86,686	$ 194,555	$ 171,347
Preferred stock dividends	(4,144)		(3,801)		(7,945)	(7,601)
Net income available to common equity	144,573		80,860		186,611	163,746
Common shareholder dividends	(27,667)		(27,571)		(55,931)	(55,650)
Unvested share-based payment awards	(222)		(205)		(351)	(308)
Undistributed earnings	116,684		53,085		130,329	107,787
Undistributed earnings allocated to common shareholders	115,750		52,690		129,377	107,098
Undistributed earnings allocated to unvested share-based payment awards	934		395		953	689
Undistributed earnings	116,684		53,085		130,329	107,787
Basic
Distributed earnings to common shareholders	27,667		27,571		55,931	55,650
Undistributed earnings allocated to common shareholders	115,750		52,690		129,377	107,098
Total common shareholders earnings, basic	143,417		80,261		185,307	162,748
Diluted
Distributed earnings to common shareholders	27,667		27,571		55,931	55,650
Undistributed earnings allocated to common shareholders	115,750		52,690		129,377	107,098
Total common shareholders earnings, diluted	$ 143,417		$ 80,261		$ 185,307	$ 162,748
Weighted average common shares outstanding	152,393		162,180		153,547	163,049
Effect of dilutive common stock awards	757		1,492		813	1,469
Diluted weighted average common shares outstanding	153,150		163,672		154,360	164,518
Basic earnings per common share	$ 0.94		$ 0.49		$ 1.21	$ 1.00
Diluted earnings per common share	$ 0.94		$ 0.49		$ 1.20	$ 0.99