Summary of Investments (Maturities) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Investments [Line Items]
Held-To-Maturity, Due in One Year or Less, Amortized Cost	$ 271
Held-To-Maturity, Due in One Year or Less, Fair Value	276
Available-For-Sale, Due in One Year or Less, Amortized Cost	261,755
Available-For-Sale, Due in One Year or Less, Fair Value	267,088
Trading, Due in One Year or Less, Amortized Cost	2,821
Trading, Due in One Year or Less, Fair Value	2,713
Held-To-Maturity, Due after One Year through Five Years, Amortized Cost	1,288
Held-To-Maturity, Due after One Year through Five Years, Fair Value	1,303
Available-For-Sale, Due after One Year through Five Years, Amortized Cost	1,028,748
Available-For-Sale, Due after One Year through Five Years, Fair Value	1,091,315
Trading, Due after One Year through Five Years, Amortized Cost	6,649
Trading, Due after One Year through Five Years, Fair Value	7,412
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	1,000,318
Available-For-Sale, Due after Five Years through Ten Years, Fair Value	1,079,653
Trading, Due after Five Years through Ten Years, Amortized Cost	0
Trading, Due after Five Years through Ten Years, Fair Value	0
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	153,105
Available-For-Sale,Due after Ten Years, Fair Value	161,613
Trading, Due after Ten Years, Amortized Cost	4,077
Trading, Due after Ten Years, Fair Value	4,373
Held-To-Maturity, Amortized Cost	1,847	4,143
Held-To-Maturity, Fair Value	1,883	4,161
Available-For-Sale, Amortized Cost	2,735,999	2,631,345
Available-For-Sale, Fair Value	3,010,431	2,856,699
Trading, Amortized Cost	13,547
Trading, Fair Value	14,498	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	4,574
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	4,986
Available-For-Sale, Amortized Cost	4,574	5,801
Available-For-Sale, Fair Value	4,986	6,296
Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Not Categorized by Contractual Maturity, Amortized Cost	267
Held-To-Maturity, Not Categorized by Contractual Maturity, Fair Value	283
Available-For-Sale, Not Categorized by Contractual Maturity, Amortized Cost	30,752
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	31,895
Held-To-Maturity, Amortized Cost	267	356
Held-To-Maturity, Fair Value	283	381
Available-For-Sale, Amortized Cost	30,752	34,353
Available-For-Sale, Fair Value	31,895	35,390
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Not Categorized by Contractual Maturity, Amortized Cost	21
Held-To-Maturity, Not Categorized by Contractual Maturity, Fair Value	21
Available-For-Sale, Not Categorized by Contractual Maturity, Amortized Cost	187,788
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	194,157
Held-To-Maturity, Amortized Cost	21	48
Held-To-Maturity, Fair Value	21	50
Available-For-Sale, Amortized Cost	187,788	79,545
Available-For-Sale, Fair Value	194,157	82,851
Debt Securities [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Amortized Cost	1,847	4,143
Held-To-Maturity, Fair Value	1,883	4,161
Available-For-Sale, Amortized Cost	2,667,040	2,562,786
Available-For-Sale, Fair Value	$ 2,830,707	$ 2,697,248