Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Calculations for basic and diluted earnings per common share
Net income	$ 333,562	$ 229,264	$ 200,274
Preferred stock dividends	(10,784)	(9,347)	(8,903)
Net income available to common equity	322,779	219,917	191,371
Common shareholder dividends	(104,981)	(75,967)	(67,100)
Unvested share-based payment awards	(537)	(450)	(755)
Undistributed earnings	217,260	143,500	123,516
Undistributed earnings allocated to common shareholders	216,199	142,593	122,205
Undistributed earnings allocated to unvested share-based payment awards	1,060	907	1,311
Undistributed earnings	217,260	143,500	123,516
Basic
Distributed earnings to common shareholders	104,981	75,967	67,100
Undistributed earnings allocated to common shareholders	216,199	142,593	122,205
Total common shareholders earnings, basic	321,181	218,560	189,305
Diluted
Distributed earnings to common shareholders	104,981	75,967	67,100
Undistributed earnings allocated to common shareholders	216,199	142,593	122,205
Total common shareholders earnings, diluted	$ 321,181	$ 218,560	$ 189,305
Weighted average common shares outstanding	167,345	150,877	148,769
Effect of dilutive common stock awards	1,985	2,038	1,192
Effect of dilutive common stock warrants	402	732	0
Diluted weighted average common shares outstanding	169,732	153,647	149,961
Basic earnings per common share	$ 1.92	$ 1.45	$ 1.27
Diluted earnings per common share	$ 1.89	$ 1.42	$ 1.26