Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Calculations for basic and diluted earnings per common share
Net income	$ 86,686	$ 69,456
Preferred stock dividends	(3,801)	(2,339)
Net income available to common equity	82,885	67,117
Common shareholder dividends	(28,080)	(25,572)
Unvested share-based payment awards	(103)	(138)
Undistributed earnings	54,702	41,407
Undistributed earnings allocated to common shareholders	54,410	41,225
Undistributed earnings allocated to unvested share-based payment awards	292	182
Undistributed earnings	54,702	41,407
Basic
Distributed earnings to common shareholders	28,080	25,572
Undistributed earnings allocated to common shareholders	54,410	41,225
Total common shareholders earnings, basic	82,490	66,797
Diluted
Distributed earnings to common shareholders	28,080	25,572
Undistributed earnings allocated to common shareholders	54,410	41,225
Total common shareholders earnings, diluted	$ 82,490	$ 66,797
Weighted average common shares outstanding	163,928	163,520
Effect of dilutive common stock awards	1,505	2,038
Effect of dilutive common stock warrants	0	874
Diluted weighted average common shares outstanding	165,433	166,432
Basic earnings per common share	$ 0.50	$ 0.41
Diluted earnings per common share	$ 0.50	$ 0.40