Held-to-Maturity Securities (Contractual Maturity) (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, Fair Value	14,058,463		13,177,117
Held To Maturity Securities Other Than Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Debt Maturities, within One Year Amortized Cost	25,579	[1]	26,238	[1]
Held-to-maturity Securities, Debt Maturities, Next Twelve Months, Fair Value			26,240
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value	25,578
Held-to-maturity Securities, Debt Maturities, After One Through Five Years Amortized Cost	0	[1]	0	[1]
Held-to-maturity Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value	0
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Fair Value			0
Held-to-maturity Securities, Debt Maturities, After Five Through Ten Years Amortized Cost	0	[1]	0	[1]
Held-to-maturity Securities, Debt Maturities, Rolling Year Six Through Ten, Fair Value	0
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Fair Value			0
Held-to-maturity Securities, Debt Maturities, After Ten Years Amortized Cost	0	[1]	0	[1]
Held-to-maturity Securities, Debt Maturities, Rolling after Ten Years, Fair Value	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Fair Value			0
Held-to-maturity Securities, Debt Maturities, Amortized Cost	25,579	[1]	26,238	[1]
Held-to-maturity Securities, Debt Maturities, Fair Value	25,578		26,240
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	25,579	[1]	26,238	[1]
Held-to-maturity Securities, Fair Value	25,578		26,240
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],[2]	12,772,210	[1],[2]
Held-to-maturity Securities, Fair Value	$ 14,032,885	[2]	$ 13,150,877	[2]

[1] Carrying value equals amortized cost.

[2] 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.