Investment Securities (Details 3) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Available for sale:
Fair value, less than 12 months	$ 870,559	$ 13,016
Unrealized losses, less than 12 months	(11,317)	(331)
Fair value, 12 months or more	40,684	43,045
Unrealized losses, 12 months or more	(13,072)	(11,609)
Fair value, total	911,243	56,061
Unrealized losses, total	(24,389)	(11,940)
Residential mortgage-backed securities
Available for sale:
Fair value, less than 12 months	857,033
Unrealized losses, less than 12 months	(11,083)
Fair value, 12 months or more	37,021	39,605
Unrealized losses, 12 months or more	(12,144)	(10,680)
Fair value, total	894,054	39,605
Unrealized losses, total	(23,227)	(10,680)
Obligations of states and political subdivisions
Available for sale:
Fair value, less than 12 months	8,550	9,531
Unrealized losses, less than 12 months	(210)	(315)
Fair value, 12 months or more	3,607	3,398
Unrealized losses, 12 months or more	(909)	(896)
Fair value, total	12,157	12,929
Unrealized losses, total	(1,119)	(1,211)
Equity Securities
Available for sale:
Fair value, less than 12 months		3,485
Unrealized losses, less than 12 months		(16)
Fair value, 12 months or more		42
Unrealized losses, 12 months or more		(33)
Fair value, total		3,527
Unrealized losses, total		(49)
Other equity securities
Available for sale:
Fair value, less than 12 months	4,976
Unrealized losses, less than 12 months	(24)
Fair value, 12 months or more	56
Unrealized losses, 12 months or more	(19)
Fair value, total	5,032
Unrealized losses, total	$ (43)