Earnings Per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Calculations for basic and diluted earnings per common share
Net income	$ 50,207	$ 45,658	$ 141,771	$ 140,934
Preferred stock dividends and discount accretion	(1,255)	(1,285)	(3,777)	(3,885)
Net income available to common equity	48,952	44,373	137,994	137,049
Common shareholder dividends	(13,983)	(13,167)	(42,864)	(39,849)
Unvested share-based payment awards	(140)	(100)	(435)	(374)
Undistributed earnings	34,829	31,106	94,695	96,826
Undistributed earnings allocated to common shareholders	34,481	30,873	93,862	96,112
Undistributed earnings allocated to unvested share-based payment awards	348	233	833	714
Undistributed earnings	34,829	31,106	94,695	96,826
Basic
Distributed earnings to common shareholders	13,983	13,167	42,864	39,849
Undistributed earnings allocated to common shareholders	34,481	30,873	93,862	96,112
Total common shareholders earnings, basic	48,464	44,040	136,726	135,961
Diluted
Distributed earnings to common shareholders	13,983	13,167	42,864	39,849
Undistributed earnings allocated to common shareholders	34,481	30,873	93,862	96,112
Total common shareholders earnings, diluted	$ 48,464	$ 44,040	$ 136,726	$ 135,961
Weighted average common shares outstanding	155,925,000	164,954,000	159,090,000	166,586,000
Effect of dilutive common stock awards	1,066,000	489,000	903,000	174,000
Diluted weighted average common shares outstanding	156,991,000	165,443,000	159,993,000	166,760,000
Basic earnings per common share	$ 0.31	$ 0.27	$ 0.86	$ 0.82
Diluted earnings per common share	$ 0.31	$ 0.27	$ 0.85	$ 0.82
Earnings Per Share (Textuals) [Abstract]
Number of antidilutive stock options excluded from the computation of diluted earnings per share	2,000,000	3,000,000	3,000,000	3,000,000