Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019
Calculations for basic and diluted earnings per common share
Net income	$ 45,214	$ 148,718	$ 45,838	$ 83,339	$ 84,661	$ 86,686	$ 239,769	$ 254,686
Preferred stock dividends	(5,207)			(3,801)			(13,152)	(11,402)
Net income available to common equity	40,007			79,539			226,618	243,285
Common shareholder dividends	(27,676)			(27,091)			(83,607)	(82,741)
Unvested share-based payment awards	(199)			(198)			(549)	(506)
Undistributed earnings	12,133			52,250			142,462	160,037
Undistributed earnings allocated to common shareholders	12,045			51,870			141,426	158,970
Undistributed earnings allocated to unvested share-based payment awards	87			380			1,036	1,067
Undistributed earnings	12,133			52,250			142,462	160,037
Basic
Distributed earnings to common shareholders	27,676			27,091			83,607	82,741
Undistributed earnings allocated to common shareholders	12,045			51,870			141,426	158,970
Total common shareholders earnings, basic	39,721			78,961			225,032	241,711
Diluted
Distributed earnings to common shareholders	27,676			27,091			83,607	82,741
Undistributed earnings allocated to common shareholders	12,045			51,870			141,426	158,970
Total common shareholders earnings, diluted	$ 39,721			$ 78,961			$ 225,032	$ 241,711
Weighted average common shares outstanding	152,440			159,126			153,175	161,727
Effect of dilutive common stock awards	754			1,256			739	1,334
Diluted weighted average common shares outstanding	153,194			160,382			153,914	163,061
Basic earnings per common share	$ 0.26			$ 0.50			$ 1.47	$ 1.49
Diluted earnings per common share	$ 0.26			$ 0.49			$ 1.46	$ 1.48