Investment Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended
	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Schedule of Investments [Line Items]
Available for sale securities, net unrealized losses	$ (40,100)	$ (44,900)	$ (65,800)
Unrealized Gain (Loss) on Investments	$ (88,400)	(102,400)	(125,200)
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	$ 43,440	52,631	56,396
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	$ 3,838,509	3,545,288	3,478,420
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss at End of Period	$ 43,440	52,631	56,396
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	$ 3,200,000	2,800,000	3,200,000
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
Collateralized Loan Obligations
Schedule of Investments [Line Items]
Available-for-sale Securities, Gross Unrealized Loss at End of Period	2,463	9,613	4,252
Available-for-sale	258,081	287,844	293,965
Available-for-sale Securities, Continuous Unrealized Loss Position, Aggregate Loss at End of Period	$ 2,463	$ 9,613	$ 4,252