Investment Securities (Details 2) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010	Jun. 30, 2010
Gross unrealized losses and fair value of securities
Fair Value, Less than 12 months	$ 321,262	$ 470,506	$ 59,116
Unrealized Losses, Less than 12 months	(1,719)	(5,139)	(150)
Number of Impaired Securities, Less than 12 months	45	196	25
Fair Value, 12 months or longer	51,560	46,189	44,358
Unrealized Losses, 12 months or longer	(10,764)	(16,133)	(17,984)
Number of Impaired Securities, 12 months or longer	11	10	10
Fair Value, Total	372,822	516,695	103,474
Unrealized Losses, Total	(12,483)	(21,272)	(18,134)
U.S. government agency debentures [Member]
Gross unrealized losses and fair value of securities
Fair Value, Less than 12 months	23,538	109,238	30,275
Unrealized Losses, Less than 12 months	(148)	(194)	(21)
Number of Impaired Securities, Less than 12 months	2	8	2
Fair Value, 12 months or longer	0	0	0
Unrealized Losses, 12 months or longer	0	0	0
Number of Impaired Securities, 12 months or longer	0	0	0
Fair Value, Total	23,538	109,238	30,275
Unrealized Losses, Total	(148)	(194)	(21)
U.S. States and political subdivisions [Member]
Gross unrealized losses and fair value of securities
Fair Value, Less than 12 months	18,176	105,530	13,822
Unrealized Losses, Less than 12 months	(158)	(2,095)	(68)
Number of Impaired Securities, Less than 12 months	24	164	20
Fair Value, 12 months or longer	655	665	689
Unrealized Losses, 12 months or longer	(23)	(24)	(13)
Number of Impaired Securities, 12 months or longer	1	1	1
Fair Value, Total	18,831	106,195	14,511
Unrealized Losses, Total	(181)	(2,119)	(81)
Residential Mortgage Backed Securities Issued By US Government Sponsored Enterprises [Member]
Gross unrealized losses and fair value of securities
Fair Value, Less than 12 months	14,229	67,474	15,016
Unrealized Losses, Less than 12 months	(87)	(544)	(60)
Number of Impaired Securities, Less than 12 months	1	7	2
Fair Value, 12 months or longer	182	195	227
Unrealized Losses, 12 months or longer	(3)	(3)	(3)
Number of Impaired Securities, 12 months or longer	2	1	1
Fair Value, Total	14,411	67,669	15,243
Unrealized Losses, Total	(90)	(547)	(63)
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Gross unrealized losses and fair value of securities
Fair Value, Less than 12 months	265,319	188,264	3
Unrealized Losses, Less than 12 months	(1,326)	(2,306)	(1)
Number of Impaired Securities, Less than 12 months	18	17	1
Fair Value, 12 months or longer	0	0	0
Unrealized Losses, 12 months or longer	0	0	0
Number of Impaired Securities, 12 months or longer	0	0	0
Fair Value, Total	265,319	188,264	3
Unrealized Losses, Total	(1,326)	(2,306)	(1)
Non-agency [Member]
Gross unrealized losses and fair value of securities
Fair Value, Less than 12 months		0
Unrealized Losses, Less than 12 months		0
Number of Impaired Securities, Less than 12 months		0
Fair Value, 12 months or longer		0
Unrealized Losses, 12 months or longer		0
Number of Impaired Securities, 12 months or longer		0
Fair Value, Total		0
Unrealized Losses, Total		0
Corporate debt securities [Member]
Gross unrealized losses and fair value of securities
Fair Value, Less than 12 months	0	0	0
Unrealized Losses, Less than 12 months	0	0	0
Number of Impaired Securities, Less than 12 months	0	0	0
Fair Value, 12 months or longer	50,723	45,329	43,442
Unrealized Losses, 12 months or longer	(10,738)	(16,106)	(17,968)
Number of Impaired Securities, 12 months or longer	8	8	8
Fair Value, Total	50,723	45,329	43,442
Unrealized Losses, Total	$ (10,738)	$ (16,106)	$ (17,968)