Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Calculations for basic and diluted earnings per common share
Net income	$ 49,400	$ 46,365	$ 96,072	$ 91,564
Preferred stock dividends	(1,545)	(1,278)	(2,773)	(2,522)
Net income available to common equity	47,855	45,087	93,299	89,042
Common shareholder dividends	(15,056)	(14,393)	(30,222)	(28,881)
Dividends on unvested share-based payment awards	(172)	(143)	(286)	(294)
Undistributed earnings	32,627	30,551	62,791	59,867
Undistributed earnings allocated to common shareholders	32,262	30,247	62,148	59,375
Undistributed earnings allocated to unvested share-based payment awards	365	304	643	492
Undistributed earnings	32,627	30,551	62,791	59,867
Basic
Distributed earnings to common shareholders	15,056	14,393	30,222	28,881
Undistributed earnings allocated to common shareholders	32,262	30,247	62,148	59,375
Total common shareholders earnings, basic	47,318	44,640	92,370	88,256
Diluted
Distributed earnings to common shareholders	15,056	14,393	30,222	28,881
Undistributed earnings allocated to common shareholders	32,262	30,247	62,148	59,375
Total common shareholders earnings, diluted	$ 47,318	$ 44,640	$ 92,370	$ 88,256
Weighted average common shares outstanding (in shares)	149,903	159,940	149,986	160,699
Effect of dilutive common stock awards (in shares)	1,205	898	1,143	814
Diluted weighted average common shares outstanding (in shares)	151,108	160,838	151,129	161,513
Basic earnings per common share (in usd per share)	$ 0.32	$ 0.28	$ 0.62	$ 0.55
Diluted earnings per common share (in usd per share)	$ 0.31	$ 0.28	$ 0.61	$ 0.55
Earnings Per Share (Textuals) [Abstract]
Number of antidilutive stock options excluded from the computation of diluted earnings per share	1,000	3,000	1,000	2,000