Investment Securities (Narrative) (Details) (USD $)	3 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014
Schedule of Investments [Line Items]
Available for sale securities, net unrealized losses	$ 10,300,000	$ (44,900,000)	$ (127,400,000)
Unrealized Gain (Loss) on Investments	(62,100,000)	(102,400,000)	(164,600,000)
Number of states with aggregate fair value of general obligation bonds greater than $10 million	11
Available-for-sale Securities, Gross Unrealized Loss at End of Period	30,770,000	52,631,000	72,093,000
Number of Single Issuer Trust Preferred Securities With Stated Maturities	8
Percent of trust preferred securities included in fair value of entire investment portfolio	1.00%
Available-for-sale	3,791,059,000	3,545,288,000	3,433,171,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss at End of Period	30,770,000	52,631,000	72,093,000
Total number of states general obligation bonds are held	37
Maximum portfolio concentration in municipal securities per corporate investment policy	20.00%
Other investment securities pledged	3,500,000,000	2,800,000,000	3,400,000,000
Amount of municipal securities with a combined individual state to total municipal outstanding equal to or less than, per corporate policy	25.00%
12 states
Schedule of Investments [Line Items]
General obligation bonds, aggregate fair value	10,000,000
Collateralized Loan Obligations
Schedule of Investments [Line Items]
Available-for-sale Securities, Gross Unrealized Loss at End of Period	4,131,000	9,613,000	3,692,000
Available-for-sale	293,962,000	287,844,000	294,559,000
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss at End of Period	$ 4,131,000	$ 9,613,000	$ 3,692,000