Summary of Investments (Maturities) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule of Investments [Line Items]
Held-To-Maturity, Due in One Year or Less, Amortized Cost	$ 594
Held-To-Maturity, Due in One Year or Less, Fair Value	597
Available-For-Sale, Due in One Year or Less, Amortized Cost	275,551
Available-For-Sale, Due in One Year or Less, Fair Value	281,662
Trading, Due in One Year or Less, Amortized Cost	2,838
Trading, Due in One Year or Less, Fair Value	2,803
Held-To-Maturity, Due after One Year through Five Years, Amortized Cost	793
Held-To-Maturity, Due after One Year through Five Years, Fair Value	808
Available-For-Sale, Due after One Year through Five Years, Amortized Cost	1,005,859
Available-For-Sale, Due after One Year through Five Years, Fair Value	1,067,382
Trading, Due after One Year through Five Years, Amortized Cost	6,439
Trading, Due after One Year through Five Years, Fair Value	7,374
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	860,281
Available-For-Sale, Due after Five Years through Ten Years, Fair Value	921,673
Trading, Due after Five Years through Ten Years, Amortized Cost	508
Trading, Due after Five Years through Ten Years, Fair Value	533
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	242,504
Available-For-Sale,Due after Ten Years, Fair Value	247,318
Trading, Due after Ten Years, Amortized Cost	2,685
Trading, Due after Ten Years, Fair Value	3,028
Held-To-Maturity, Amortized Cost	1,638	1,655
Held-to-Maturity, Fair Value	1,668	1,681
Available-For-Sale, Amortized Cost	2,734,366	2,724,692
Available-for-Sale, Fair Value	3,003,259	2,985,205
Trading, Amortized Cost	12,470
Trading Securities, Debt	13,738	13,353
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	4,044
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	4,474
Available-For-Sale, Amortized Cost	4,044	4,480
Available-for-Sale, Fair Value	4,474	4,886
Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Not Categorized by Contractual Maturity, Amortized Cost	242
Held-To-Maturity, Not Categorized by Contractual Maturity, Fair Value	254
Available-For-Sale, Not Categorized by Contractual Maturity, Amortized Cost	24,808
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	25,538
Held-To-Maturity, Amortized Cost	242	256
Held-to-Maturity, Fair Value	254	271
Available-For-Sale, Amortized Cost	24,808	27,940
Available-for-Sale, Fair Value	25,538	28,807
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Not Categorized by Contractual Maturity, Amortized Cost	9
Held-To-Maturity, Not Categorized by Contractual Maturity, Fair Value	9
Available-For-Sale, Not Categorized by Contractual Maturity, Amortized Cost	253,855
Available-For-Sale,Not Categorized by Contractual Maturity, Fair Value	259,535
Held-To-Maturity, Amortized Cost	9	14
Held-to-Maturity, Fair Value	9	14
Available-For-Sale, Amortized Cost	253,855	208,042
Available-for-Sale, Fair Value	259,535	214,584
Debt Securities [Member]
Schedule of Investments [Line Items]
Held-To-Maturity, Amortized Cost	1,638	1,655
Held-to-Maturity, Fair Value	1,668	1,681
Available-For-Sale, Amortized Cost	2,666,902	2,657,800
Available-for-Sale, Fair Value	$ 2,807,582	$ 2,808,078