Held-to-Maturity Securities (Major Security Types) (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	[3]	14,687,378,000	14,546,979,000
Held-to-maturity Securities, Unrecognized Holding Gain		57,202,000	40,803,000
Held-to-maturity Securities, Unrecognized Holding Loss		(110,467,000)	(174,551,000)
Held-to-maturity Securities, Fair Value		14,634,113,000	14,413,231,000
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	[2]	32,789,000	31,279,000
Held-to-maturity Securities		32,789,000	31,279,000
Held-to-maturity Securities, Unrecognized Holding Gain		0	1,000
Held-to-maturity Securities, Unrecognized Holding Loss		(3,000)	0
Held-to-maturity Securities, Fair Value		32,786,000	31,280,000
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	[1],[2]	32,789,000	31,279,000
Held-to-maturity Securities		32,789,000	31,279,000
Held-to-maturity Securities, Unrecognized Holding Gain		0	1,000
Held-to-maturity Securities, Unrecognized Holding Loss		(3,000)	0
Held-to-maturity Securities, Fair Value		32,786,000	31,280,000
Single Family, 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	[2]	2,881,992,000	3,183,219,000
Held-to-maturity Securities		2,881,992,000	3,183,219,000
Held-to-maturity Securities, Unrecognized Holding Gain		4,998,000	3,653,000
Held-to-maturity Securities, Unrecognized Holding Loss		(15,951,000)	(23,151,000)
Held-to-maturity Securities, Fair Value		2,871,039,000	3,163,721,000
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	[4]	14,654,589,000	14,515,700,000
Held-to-maturity Securities, Unrecognized Holding Gain		57,202,000	40,802,000
Held-to-maturity Securities, Unrecognized Holding Loss		(110,464,000)	(174,551,000)
Held-to-maturity Securities, Fair Value	[4]	14,601,327,000	14,381,951,000
Single Family [Member] | 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	[2]	7,457,569,000	8,186,733,000
Held-to-maturity Securities		7,457,569,000	8,186,733,000
Held-to-maturity Securities, Unrecognized Holding Gain		44,301,000	36,161,000
Held-to-maturity Securities, Unrecognized Holding Loss		(94,278,000)	(147,494,000)
Held-to-maturity Securities, Fair Value		7,407,592,000	8,075,400,000
Multifamily [Member] | 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	[2]	4,315,028,000	3,145,748,000
Held-to-maturity Securities		4,315,028,000	3,145,748,000
Held-to-maturity Securities, Unrecognized Holding Gain		7,903,000	988,000
Held-to-maturity Securities, Unrecognized Holding Loss		(235,000)	(3,906,000)
Held-to-maturity Securities, Fair Value		$ 4,322,696,000	$ 3,142,830,000

[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.