Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Calculations for basic and diluted earnings per common share
Net income	$ 56,270	$ 42,534
Preferred stock dividends	(2,330)	(2,198)
Net income available to common equity	53,940	40,336
Common shareholder dividends	(18,251)	(16,203)
Unvested share-based payment awards	(117)	(206)
Undistributed earnings	35,572	23,927
Undistributed earnings allocated to common shareholders	35,294	23,686
Undistributed earnings allocated to unvested share-based payment awards	278	241
Undistributed earnings	35,572	23,927
Basic
Distributed earnings to common shareholders	18,251	16,203
Undistributed earnings allocated to common shareholders	35,294	23,686
Total common shareholders earnings, basic	53,545	39,889
Diluted
Distributed earnings to common shareholders	18,251	16,203
Undistributed earnings allocated to common shareholders	35,294	23,686
Total common shareholders earnings, diluted	$ 53,545	$ 39,889
Weighted average common shares outstanding (in shares)	150,815	148,601
Effect of dilutive common stock awards (in shares)	3,054	853
Diluted weighted average common shares outstanding (in shares)	153,869	149,454
Basic earnings (loss) per common share (in usd per share)	$ 0.36	$ 0.27
Diluted earnings (loss) per common share (in usd per share)	$ 0.35	$ 0.27