Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Calculations for basic and diluted earnings per common share
Net income	$ 49,438	$ 50,207	$ 145,510	$ 141,771
Preferred stock dividends	(2,184)	(1,255)	(4,957)	(3,777)
Net income available to common equity	47,254	48,952	140,553	137,994
Common shareholder dividends	(14,927)	(13,983)	(45,149)	(42,864)
Dividends on unvested share-based payment awards	(164)	(140)	(450)	(435)
Undistributed earnings	32,163	34,829	94,954	94,695
Undistributed earnings allocated to common shareholders	31,813	34,481	93,961	93,862
Undistributed earnings allocated to unvested share-based payment awards	350	348	993	833
Undistributed earnings	32,163	34,829	94,954	94,695
Basic
Distributed earnings to common shareholders	14,927	13,983	45,149	42,864
Undistributed earnings allocated to common shareholders	31,813	34,481	93,961	93,862
Total common shareholders earnings, basic	46,740	48,464	139,110	136,726
Diluted
Distributed earnings to common shareholders	14,927	13,983	45,149	42,864
Undistributed earnings allocated to common shareholders	31,813	34,481	93,961	93,862
Total common shareholders earnings, diluted	$ 46,740	$ 48,464	$ 139,110	$ 136,726
Weighted average common shares outstanding (in shares)	148,614	155,925	149,524	159,090
Effect of dilutive common stock awards (in shares)	1,185	1,066	1,180	903
Diluted weighted average common shares outstanding (in shares)	149,799	156,991	150,704	159,993
Basic earnings per common share (in usd per share)	$ 0.31	$ 0.31	$ 0.93	$ 0.86
Diluted earnings per common share (in usd per share)	$ 0.31	$ 0.31	$ 0.92	$ 0.85
Earnings Per Share (Textuals) [Abstract]
Number of antidilutive stock options excluded from the computation of diluted earnings per share	1,000	2,000	1,000	3,000