Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of hedging activities recorded in interest expense CO bonds		$ 598,585	$ 545,228		$ 421,622
Amortization of hedging activities recorded in net gains (losses) on derivatives and hedging activities			2,336		551
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		3,607,359	3,265,130		3,244,778
Other comprehensive income (loss)		129,710	10,433		56,574
Period end		3,145,312	3,607,359		3,265,130
Net Unrealized Loss Relating to Hedging Activities [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(29,119)	(40,436)		(48,187)
Cumulative effect of change in accounting principle		(175)
Net Unrealized Gains (Losses)		(6,131)	7,907		(2,838)
Amortization - hedging activities			3,410	[1]	10,589	[2]
Amortization - hedging activities	[3]	5,218
Other comprehensive income (loss)		(913)	11,317		7,751
Period end		(30,207)	(29,119)		(40,436)
Pension and Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(5,276)	(5,955)		(6,139)
Net actuarial (loss) gain		(2,195)	(670)		(586)
Amortization - pension and postretirement benefits	[4]	664	1,349		770
Other comprehensive income (loss)		(1,531)	679		184
Period end		(6,807)	(5,276)		(5,955)
Total [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(316,507)	(326,940)		(383,514)
Cumulative effect of change in accounting principle		(175)
Net Unrealized Gains (Losses)		72,905	(22,720)		11,640
Noncredit other-than-temporary impairment losses		(181)
Noncredit losses included in basis of securities sold		29,517
Accretion of noncredit loss		22,806	28,834		32,815
Net actuarial (loss) gain		(2,195)	(670)		(586)
Noncredit other-than-temporary impairment losses reclassified to credit loss	[5]	976	230		1,346
Amortization - hedging activities			3,410	[1]	10,589	[2]
Amortization - hedging activities	[3]	5,218
Amortization - pension and postretirement benefits	[4]	664	1,349		770
Other comprehensive income (loss)		129,710	10,433		56,574
Period end		(186,972)	(316,507)		(326,940)
Available-for-sale Securities [Member] | Net Unrealized Loss on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(152,958)	(122,331)		(136,809)
Net Unrealized Gains (Losses)		79,036	(30,627)		14,478
Other comprehensive income (loss)		79,036	(30,627)		14,478
Period end		(73,922)	(152,958)		(122,331)
Held-to-maturity Securities [Member] | Noncredit Portion of Other-Than-Temporary Impairment Losses on Held-to-Maturity Securities [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(129,154)	(158,218)		(192,379)
Noncredit other-than-temporary impairment losses		(181)
Noncredit losses included in basis of securities sold		29,517
Accretion of noncredit loss		22,806	28,834		32,815
Noncredit other-than-temporary impairment losses reclassified to credit loss	[5]	976	230		1,346
Other comprehensive income (loss)		53,118	29,064		34,161
Period end		$ (76,036)	(129,154)		(158,218)
Interest Rate Contract [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] | Net Unrealized Loss Relating to Hedging Activities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of hedging activities recorded in interest expense CO bonds			3,200		10,600
Amortization of hedging activities recorded in net gains (losses) on derivatives and hedging activities			$ 239		$ 14

[1] Amortization of hedging activities includes $3.2 million recorded in CO bond interest expense and $239 thousand recorded in net gains on derivatives and hedging activities in the statement of operations.

[2] Amortization of hedging activities includes $10.6 million recorded in CO bond interest expense and $14 thousand recorded in net gains on derivatives and hedging activities in the statement of operations.

[3] Recorded in CO bond interest expense.

[4] Recorded in other expenses in the statement of operations.

[5] Recorded in net other-than-temporary impairment losses on investment securities, credit portion in the statement of operations.