Investment Securities (Narrative) (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Jun. 30, 2016 USD ($) state	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
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 | state	11
Total number of states general obligation bonds are held | state	37
Available for sale securities, net unrealized losses	$ 76,300	$ 40,100	$ (53,000)
Unrealized gain (loss) on investments	$ (27,900)	(88,400)	(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	$ 4,318,688	3,838,509	3,967,735
Other investment securities pledged	$ 3,200,000	3,200,000	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	$ 291,794	$ 258,081	$ 289,411
Accumulated unrealized gain (loss), before tax	$ (6,100)