Investments AFS - Interest Rate Payment Terms Table (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
AFS Securities, Amortized Cost	[1]	$ 10,010,280	$ 11,000,910
Non-MBS [Member]
Debt Securities, Available-for-sale [Line Items]
AFS Securities, Amortized Cost		1,841,453	1,746,760
MBS [Member]
Debt Securities, Available-for-sale [Line Items]
AFS Securities, Amortized Cost		8,168,827	9,254,150	[1]
Fixed Interest Rate [Member] | Non-MBS [Member]
Debt Securities, Available-for-sale [Line Items]
AFS Securities, Amortized Cost		1,841,453	1,746,760
Fixed Interest Rate [Member] | MBS [Member]
Debt Securities, Available-for-sale [Line Items]
AFS Securities, Amortized Cost		1,619,573	1,444,111
Variable interest rate [Member] | Non-MBS [Member]
Debt Securities, Available-for-sale [Line Items]
AFS Securities, Amortized Cost		0	0
Variable interest rate [Member] | MBS [Member]
Debt Securities, Available-for-sale [Line Items]
AFS Securities, Amortized Cost		$ 6,549,254	$ 7,810,039

[1] Includes adjustments made to the cost basis of an investment for accretion, amortization, OTTI and/or fair value hedge accounting adjustments, and excludes accrued interest receivable of $19.3 million and $24.4 million at September 30, 2020 and December 31, 2019.