Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Calculations for basic and diluted earnings per common share
Net income	$ 229,264	$ 200,274	$ 188,301
Preferred stock dividends	(9,347)	(8,903)	(7,155)
Net income available to common equity	219,917	191,371	181,146
Common shareholder dividends	(75,967)	(67,100)	(61,774)
Unvested share-based payment awards	(450)	(755)	(626)
Undistributed earnings	143,500	123,516	118,746
Undistributed earnings allocated to common shareholders	142,593	122,205	117,498
Undistributed earnings allocated to unvested share-based payment awards	907	1,311	1,248
Undistributed earnings	143,500	123,516	118,746
Basic
Distributed earnings to common shareholders	75,967	67,100	61,774
Undistributed earnings allocated to common shareholders	142,593	122,205	117,498
Total common shareholders earnings, basic	218,560	189,305	179,272
Diluted
Distributed earnings to common shareholders	75,967	67,100	61,774
Undistributed earnings allocated to common shareholders	142,593	122,205	117,498
Total common shareholders earnings, diluted	$ 218,560	$ 189,305	$ 179,272
Weighted average common shares outstanding	150,877	148,769	149,350
Effect of dilutive common stock awards	2,038	1,192	1,253
Effect of dilutive common stock warrants	732	0	0
Diluted weighted average common shares outstanding	153,647	149,961	150,603
Basic earnings per common share	$ 1.45	$ 1.27	$ 1.20
Diluted earnings per common share	$ 1.42	$ 1.26	$ 1.19