Held-to-Maturity Securities (Major Security Types) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013		Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	$ 13,720,233	[1]	$ 12,798,448	[1]
Held-to-maturity Securities, Unrecognized Holding Gain	342,333		379,694
Held-to-maturity Securities, Unrecognized Holding Loss	(4,103)		(1,025)
Held-to-maturity Securities, Fair Value	14,058,463		13,177,117
US Government-sponsored Enterprises Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	25,579	[1],[2]	26,238	[1],[2]
Held-to-maturity Securities, Unrecognized Holding Gain	0	[2]	2	[2]
Held-to-maturity Securities, Unrecognized Holding Loss	(1)	[2]	0	[2]
Held-to-maturity Securities, Fair Value	25,578	[2]	26,240	[2]
Held To Maturity Securities Other Than Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	25,579	[1]	26,238	[1]
Held-to-maturity Securities, Unrecognized Holding Gain	0		2
Held-to-maturity Securities, Unrecognized Holding Loss	(1)		0
Held-to-maturity Securities, Fair Value	25,578		26,240
Mortgage Backed Securities Other US Obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	1,357,198	[1],[3]	1,410,720	[1],[3]
Held-to-maturity Securities, Unrecognized Holding Gain	7,349	[3]	4,320	[3]
Held-to-maturity Securities, Unrecognized Holding Loss	0	[3]	0	[3]
Held-to-maturity Securities, Fair Value	1,364,547	[3]	1,415,040	[3]
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	12,337,456	[1],[4]	11,361,490	[1],[4]
Held-to-maturity Securities, Unrecognized Holding Gain	334,984	[4]	375,372	[4]
Held-to-maturity Securities, Unrecognized Holding Loss	(4,102)	[4]	(1,025)	[4]
Held-to-maturity Securities, Fair Value	12,668,338	[4]	11,735,837	[4]
Collateralized Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	13,694,654	[1],[5]	12,772,210	[1],[5]
Held-to-maturity Securities, Unrecognized Holding Gain	342,333		379,692
Held-to-maturity Securities, Unrecognized Holding Loss	(4,102)		(1,025)
Held-to-maturity Securities, Fair Value	$ 14,032,885	[5]	$ 13,150,877	[5]

[1] Carrying value equals amortized cost.

[2] Consists of debt securities issued and effectively guaranteed by Freddie Mac and/or Fannie Mae, which have the support of the U.S. government, although they are not obligations of the U.S. government.

[3] Consists of mortgage-backed securities issued or guaranteed by the National Credit Union Administration (NCUA) and the U.S. government.

[4] Consists of mortgage-backed securities issued and effectively guaranteed by Freddie Mac and/or Fannie Mae, which have the support of the U.S. government, although they are not obligations of the U.S. government.

[5] Mortgage-backed securities are not presented by contractual maturity because their expected maturities will likely differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment fees.