Held-to-Maturity Securities (Contractual Maturity) (Details) - USD ($)		Jun. 30, 2017	Dec. 31, 2016
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	[1],[2]	$ 14,687,378,000	$ 14,546,979,000
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	[1],[2]	14,687,378,000	14,546,979,000
Held-to-maturity Securities	[3]	14,687,378,000	14,546,979,000
Held-to-maturity Securities, Fair Value		14,634,113,000	14,413,231,000
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	[1]	32,789,000	31,279,000
Held-to-maturity Securities, Debt Maturities, After One Through Five Years Amortized Cost	[1]	0	0
Held-to-maturity Securities, Debt Maturities, After Five Through Ten Years Amortized Cost	[1]	0	0
Held-to-maturity Securities, Debt Maturities, After Ten Years Amortized Cost	[1]	0	0
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	[1],[2]	32,789,000	31,279,000
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	[1],[2]	32,789,000	31,279,000
Held-to-maturity Securities, Debt Maturities, within One Year, Net Carrying Amount		32,789,000	31,279,000
Held-to-maturity Securities, Debt Maturities, after One Through Five Years, Net Carrying Amount		0	0
Held-to-maturity Securities, Debt Maturities, after Five Through Ten Years, Net Carrying Amount		0	0
Held-to-maturity Securities, Debt Maturities, after Ten Years, Net Carrying Amount		0	0
Held-to-maturity Securities		32,789,000	31,279,000
Held-to-maturity Securities, Debt Maturities, within One Year, Fair Value		32,786,000	31,280,000
Held-to-maturity Securities, Debt Maturities, Rolling Year Two Through Five, Fair Value		0	0
Held-to-maturity Securities, Debt Maturities, Rolling Year Six Through Ten, Fair Value		0	0
Held-to-maturity Securities, Debt Maturities, Rolling after Ten Years, Fair Value		0	0
Held-to-maturity Securities, Fair Value		32,786,000	31,280,000
Collateralized Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	[1],[2],[4]	14,654,589,000	14,515,700,000
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	[1],[2],[4]	14,654,589,000	14,515,700,000
Held-to-maturity Securities	[4]	14,654,589,000	14,515,700,000
Held-to-maturity Securities, Fair Value	[4]	$ 14,601,327,000	$ 14,381,951,000

[1] Carrying value equals amortized cost.

[2] Carrying value equals amortized cost.

[3] Fair values: $14,634,113 and $14,413,231 at June 30, 2017 and December 31, 2016, respectively.

[4] 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.