Investment Securities (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Schedule of Investments [Line Items]
General obligation bonds, aggregate fair value	$ 10,000
Number of states with aggregate fair value of general obligation bonds greater than $10 million	11
Total number of states general obligation bonds are held	37
Available for sale securities, net unrealized losses	$ 33,900	$ (10,300)	$ (53,000)
Unrealized gain (loss) on investments	$ (50,300)	(62,100)	(100,400)
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	$ 4,104,214	3,791,059	3,967,735
Other investment securities pledged	$ 3,400,000	3,500,000	2,900,000
Maximum portfolio concentration in municipal securities per corporate investment policy	20.00%
Amount of municipal securities with a combined individual state to total municipal outstanding equal to or less than, per corporate policy	25.00%
Collateralized Loan Obligations
Schedule of Investments [Line Items]
Available-for-sale	$ 285,397	$ 293,962	$ 289,411
Accumulated unrealized gain (loss), before tax	$ (12,500)