Investments HTM - Securities by Major Security Type Table (Details) - USD ($) $ in Thousands		Sep. 30, 2020		Dec. 31, 2019
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost (1)	[1]	$ 1,967,036		$ 2,395,691
Gross Unrealized Holding Gains		82,883		49,095
Gross Unrealized Holding Losses		(2,565)		(4,498)
HTM securities - fair value		2,047,354		2,440,288
Debt Securities, Held-to-Maturity, Accrued Interest, after Allowance for Credit Loss		4,500		6,200
State or local agency obligations [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost (1)				94,310
Gross Unrealized Holding Gains				0
Gross Unrealized Holding Losses				(3,394)
HTM securities - fair value				90,916
U.S. obligations single-family MBS
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost (1)		176,039	[1]	250,195
Gross Unrealized Holding Gains		1,384		1,087
Gross Unrealized Holding Losses		0		(78)
HTM securities - fair value		177,423		251,204
Private label MBS [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost (1)		101,327	[1]	123,818
Gross Unrealized Holding Gains		301		881
Gross Unrealized Holding Losses		(1,856)		(520)
HTM securities - fair value		99,772		124,179
MBS [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost (1)		1,967,036	[1]	2,301,381
Gross Unrealized Holding Gains		82,883		49,095
Gross Unrealized Holding Losses		(2,565)		(1,104)
HTM securities - fair value		2,047,354		2,349,372
Single Family [Member] | GSE MBS [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost (1)		1,158,572	[1]	1,156,545
Gross Unrealized Holding Gains		26,140		20,896
Gross Unrealized Holding Losses		(709)		(254)
HTM securities - fair value		1,184,003		1,177,187
Multifamily [Member] | GSE MBS [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost (1)		531,098	[1]	770,823
Gross Unrealized Holding Gains		55,058		26,231
Gross Unrealized Holding Losses		0		(252)
HTM securities - fair value		$ 586,156		$ 796,802

[1] Includes adjustments made to the cost basis of an investment for accretion and amortization and excludes accrued interest receivable of $4.5 million and $6.2 million at September 30, 2020 and December 31, 2019.