Investment Securities AFS Securities Amortized Cost and Fair Value (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Investment securities available for sale:
Investment securities available for sale, amortized cost	$ 5,329,821	$ 5,270,576
Investment securities available for sale, gross unrealized gains	87,096	88,043
Investment securities available for sale, gross unrealized losses	(71,495)	(108,034)
Investment securities available for sale, fair value	5,345,422	5,250,585
US Treasury Securities [Member]
Investment securities available for sale:
Investment securities available for sale, amortized cost	999	1,001
Investment securities available for sale, gross unrealized gains	0	1
Investment securities available for sale, gross unrealized losses	(3)	0
Investment securities available for sale, fair value	996	1,002
Obligations of state and political subdivisions (municipal securities) [Member]
Investment securities available for sale:
Investment securities available for sale, amortized cost	588,255	653,758
Investment securities available for sale, gross unrealized gains	26,229	23,855
Investment securities available for sale, gross unrealized losses	(48)	(1,533)
Investment securities available for sale, fair value	614,436	676,080
Government-Sponsored Enterprise (GSE) [Member]
Investment securities available for sale:
Investment securities available for sale, amortized cost	3,725,218	3,855,467
Investment securities available for sale, gross unrealized gains	59,047	61,542
Investment securities available for sale, gross unrealized losses	(43,225)	(78,579)
Investment securities available for sale, fair value	3,741,040	3,838,430
Private Label [Member]
Investment securities available for sale:
Investment securities available for sale, amortized cost	2,475	3,035
Investment securities available for sale, gross unrealized gains	9	16
Investment securities available for sale, gross unrealized losses	(2)	(37)
Investment securities available for sale, fair value	2,482	3,014
GNMA commercial mortgage-related securities [Member]
Investment securities available for sale:
Investment securities available for sale, amortized cost	1,007,068	673,555
Investment securities available for sale, gross unrealized gains	1,751	1,764
Investment securities available for sale, gross unrealized losses	(28,217)	(27,842)
Investment securities available for sale, fair value	980,602	647,477
Asset-backed Securities [Member]
Investment securities available for sale:
Investment securities available for sale, amortized cost	0	23,049
Investment securities available for sale, gross unrealized gains	0	10
Investment securities available for sale, gross unrealized losses	0	0
Investment securities available for sale, fair value	0	23,059
Other Debt And Other Equity Securities [Member]
Investment securities available for sale:
Investment securities available for sale, amortized cost	5,806	60,711
Investment securities available for sale, gross unrealized gains	60	855
Investment securities available for sale, gross unrealized losses	0	(43)
Investment securities available for sale, fair value	$ 5,866	$ 61,523