Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Calculations for basic and diluted earnings per common share
Net income	$ 69,456	$ 56,270
Preferred stock dividends	(2,339)	(2,330)
Net income available to common equity	67,117	53,940
Common shareholder dividends	(25,572)	(18,251)
Unvested share-based payment awards	(138)	(117)
Undistributed earnings	41,407	35,572
Undistributed earnings allocated to common shareholders	41,225	35,294
Undistributed earnings allocated to unvested share-based payment awards	182	278
Undistributed earnings	41,407	35,572
Basic
Distributed earnings to common shareholders	25,572	18,251
Undistributed earnings allocated to common shareholders	41,225	35,294
Total common shareholders earnings, basic	66,797	53,545
Diluted
Distributed earnings to common shareholders	25,572	18,251
Undistributed earnings allocated to common shareholders	41,225	35,294
Total common shareholders earnings, diluted	$ 66,797	$ 53,545
Weighted average common shares outstanding (in shares)	163,520	150,815
Effect of dilutive common stock awards (in shares)	2,038	2,216
Effect of dilutive common stock warrants (in shares)	874	838
Diluted weighted average common shares outstanding (in shares)	166,432	153,869
Basic earnings (loss) per common share (in usd per share)	$ 0.41	$ 0.36
Diluted earnings (loss) per common share (in usd per share)	$ 0.40	$ 0.35