Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Calculations for basic and diluted earnings per common share
Net income	$ 49,091	$ 49,400	$ 91,625	$ 96,072
Preferred stock dividends	(2,169)	(1,545)	(4,367)	(2,773)
Net income available to common equity	46,922	47,855	87,258	93,299
Common shareholder dividends	(16,443)	(15,056)	(32,646)	(30,222)
Unvested share-based payment awards	(182)	(172)	(388)	(286)
Undistributed earnings	30,297	32,627	54,224	62,791
Undistributed earnings allocated to common shareholders	29,963	32,262	53,649	62,148
Undistributed earnings allocated to unvested share-based payment awards	334	365	575	643
Undistributed earnings	30,297	32,627	54,224	62,791
Basic
Distributed earnings to common shareholders	16,443	15,056	32,646	30,222
Undistributed earnings allocated to common shareholders	29,963	32,262	53,649	62,148
Total common shareholders earnings, basic	46,406	47,318	86,295	92,370
Diluted
Distributed earnings to common shareholders	16,443	15,056	32,646	30,222
Undistributed earnings allocated to common shareholders	29,963	32,262	53,649	62,148
Total common shareholders earnings, diluted	$ 46,406	$ 47,318	$ 86,295	$ 92,370
Weighted average common shares outstanding (in shares)	148,511	149,903	148,556	149,986
Effect of dilutive common stock awards (in shares)	1,019	1,205	962	1,143
Diluted weighted average common shares outstanding (in shares)	149,530	151,108	149,518	151,129
Basic earnings (loss) per common share (in usd per share)	$ 0.31	$ 0.32	$ 0.58	$ 0.62
Diluted earnings (loss) per common share (in usd per share)	$ 0.31	$ 0.31	$ 0.58	$ 0.61
Earnings Per Share (Textuals) [Abstract]
Number of antidilutive stock options excluded from the computation of diluted earnings per share (in shares)	1,000	1,000	2,000	1,000
Number of antidilutive warrants excluded from the computation of diluted earnings per share (in shares)	4,000	4,000	4,000	4,000