Investment Securities (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Available for sale:
Fair value, less than 12 months	$ 2,157,150	$ 743,609
Unrealized losses, less than 12 months	(16,201)	(5,590)
Fair value, 12 months or more	41,012	43,353
Unrealized losses, 12 months or more	(13,125)	(12,254)
Fair value, total	2,198,162	786,962
Unrealized losses, total	(29,326)	(17,844)
Residential mortgage-backed securities
Available for sale:
Fair value, less than 12 months	2,140,381	738,492
Unrealized losses, less than 12 months	(15,738)	(5,476)
Fair value, 12 months or more	30,697	32,860
Unrealized losses, 12 months or more	(11,215)	(10,557)
Fair value, total	2,171,078	771,352
Unrealized losses, total	(26,953)	(16,033)
Obligations of states and political subdivisions
Available for sale:
Fair value, less than 12 months	12,539	5,117
Unrealized losses, less than 12 months	(442)	(114)
Fair value, 12 months or more	10,250	10,437
Unrealized losses, 12 months or more	(1,900)	(1,677)
Fair value, total	22,789	15,554
Unrealized losses, total	(2,342)	(1,791)
Other equity securities
Available for sale:
Fair value, less than 12 months	4,230
Unrealized losses, less than 12 months	(21)
Fair value, 12 months or more	65
Unrealized losses, 12 months or more	(10)
Fair value, total	4,295
Unrealized losses, total	(31)
Equity Securities
Available for sale:
Fair value, 12 months or more		56
Unrealized losses, 12 months or more		(20)
Fair value, total		56
Unrealized losses, total		$ (20)