Summary of Investments (Maturities) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Held-To-Maturity, Due in One Year or Less, Amortized Cost	$ 406
Held-To-Maturity, Due in One Year or Less, Fair Value	416
Available-For-Sale, Due in One Year or Less, Amortized Cost	241,061
Available-For-Sale, Due in One Year or Less, Fair Value	245,155
Trading, Due in One Year or Less, Amortized Cost	490
Trading, Due in One Year or Less, Fair Value	520
Held-To-Maturity, Due after One Year through Five Years, Amortized Cost	3,150
Held-To-Maturity, Due after One Year through Five Years, Fair Value	3,184
Available-For-Sale, Due after One Year through Five Years, Amortized Cost	1,051,843
Available-For-Sale, Due after One Year through Five Years, Fair Value	1,107,127
Trading, Due after One Year through Five Years, Amortized Cost	7,376
Trading, Due after One Year through Five Years, Fair Value	7,171
Held-To-Maturity, Due after Five Years through Ten Years, Amortized Cost	0
Held-To-Maturity, Due after Five Years through Ten Years, Fair Value	0
Available-For-Sale, Due after Five Years through Ten Years, Amortized Cost	1,054,621
Available-For-Sale, Due after Five Years through Ten Years, Fair Value	1,123,705
Trading, Due after Five Years through Ten Years, Amortized Cost	1,844
Trading, Due after Five Years through Ten Years, Fair Value	1,776
Held-To-Maturity, Due after Ten Years, Amortized Cost	0
Held-To-Maturity, Due after Ten Years, Fair Value	0
Available-For-Sale, Due after Ten Years, Amortized Cost	141,826
Available-For-Sale,Due after Ten Years, Fair Value	148,318
Trading, Due after Ten Years, Amortized Cost	4,553
Trading, Due after Ten Years, Fair Value	4,782
Held-To-Maturity, Amortized Cost	3,864	4,143
Held-To-Maturity, Fair Value	3,927	4,161
Available-For-Sale, Amortized Cost	2,726,601	2,631,345
Available-For-Sale, Fair Value	2,974,983	2,856,699
Trading, Amortized Cost	14,263
Trading, Fair Value	14,249	13,454
Asset-Backed Securities [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Not Categorized by Contractual Maturity, Amortized Cost	0
Held-To-Maturity, Not Categorized by Contractual Maturity, Fair Value	0
Available-For-Sale, Not Categorized by Contractual Maturity, Amortized Cost	5,188
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	5,469
Available-For-Sale, Amortized Cost	5,188	5,801
Available-For-Sale, Fair Value	5,469	6,296
Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Not Categorized by Contractual Maturity, Amortized Cost	280
Held-To-Maturity, Not Categorized by Contractual Maturity, Fair Value	298
Available-For-Sale, Not Categorized by Contractual Maturity, Amortized Cost	33,884
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	34,992
Held-To-Maturity, Amortized Cost	280	356
Held-To-Maturity, Fair Value	298	381
Available-For-Sale, Amortized Cost	33,884	34,353
Available-For-Sale, Fair Value	34,992	35,390
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Not Categorized by Contractual Maturity, Amortized Cost	28
Held-To-Maturity, Not Categorized by Contractual Maturity, Fair Value	29
Available-For-Sale, Not Categorized by Contractual Maturity, Amortized Cost	129,515
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	135,858
Held-To-Maturity, Amortized Cost	28	48
Held-To-Maturity, Fair Value	29	50
Available-For-Sale, Amortized Cost	129,515	79,545
Available-For-Sale, Fair Value	135,858	82,851
Debt Securities [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Amortized Cost	3,864	4,143
Held-To-Maturity, Fair Value	3,927	4,161
Available-For-Sale, Amortized Cost	2,657,938	2,562,786
Available-For-Sale, Fair Value	$ 2,800,624	$ 2,697,248