Investments AFS - Contractual Maturities Table (Details) - USD ($) $ in Thousands		Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost (1)	[1]	$ 10,010,280	$ 11,000,910
AFS Securities		10,153,022	11,097,769
Non-MBS [Member]
Debt Securities, Available-for-sale [Line Items]
AFS - Non-MBS - Due in one year or less - Amortized Cost		65,371	0
AFS - Non-MBS - Due after one year through five years - Amortized Cost		545,669	525,301
AFS - Non-MBS - Due after five through ten years - Amortized Cost		795,744	700,613
AFS - Non-MBS - Due after 10 years - Amortized Cost		434,669	520,846
Amortized Cost (1)		1,841,453	1,746,760
AFS - Non-MBS - Due in one year or less - Fair Value		65,586	0
AFS - Non-MBS - Due after one year through five years - Fair Value		557,360	534,642
AFS - Non-MBS - Due after five through ten years - Fair Value		823,978	719,672
AFS - Non-MBS - Due after 10 years - Fair Value		459,637	544,279
AFS Securities		1,906,561	1,798,593
MBS [Member]
Debt Securities, Available-for-sale [Line Items]
Amortized Cost (1)		8,168,827	9,254,150	[1]
AFS Securities		$ 8,246,461	$ 9,299,176

[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.