Credit Facilities (Narrative) (Details)				9 Months Ended	12 Months Ended
	Aug. 31, 2020 USD ($)	May 26, 2016 USD ($)	Jun. 22, 2012 USD ($)	Sep. 30, 2020 GBP (£) Item Rate	Dec. 31, 2018	Sep. 30, 2020 USD ($) Rate	Mar. 25, 2020 USD ($)	Dec. 31, 2019 GBP (£)	Dec. 31, 2019 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, number of facilities | Item				2
Line of credit facility, commitment				£ 52,175,000		$ 1,000,000,000
Maximum percentage of admitted assets allowed for borrowing base				10.00%		10.00%
Admitted assets for FHLBNY						$ 14,667,099,000
Borrowings from FHLBNY	$ 90,000,000					90,000,000
Maximum amount available under FHLB						1,466,709,000
FHLBNY interest rate	0.35%
FHLBNY maturity date				Nov. 30, 2020
FHLBNY amount of borrowed funds required to be used to acquire additional membership stock, percentage	4.50%
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment		$ 800,000,000	$ 800,000,000			$ 800,000,000			800,000,000
Line of credit facility, term of credit facility		5 years	4 years
Line of credit facility, required minimum debt-to-capital ratio | Rate				35.00%		35.00%
Line of credit facility, required minimum net worth						$ 5,370,979,000
Percentage amount of consolidated net income required to be reserved to maintain compliance with facility covenants				25.00%
Percentage amount of increase in consolidated net worth from new issuance required to be reserved to maintain compliance with facility covenants				25.00%
Increase in reserves due to new issuance during the period required to maintain compliance with facility covenants						6,372,662,000
Revolving credit borrowings						685,263,000			623,484,000
Wells Fargo Bank Group Credit Facility Tranche One [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment						200,000,000
Revolving credit borrowings							$ 50,000,000		0
Wells Fargo Bank Group Credit Facility Tranche One [Member] | Federal Funds [Member]
Line of Credit Facility [Line Items]
Line of credit facility, spread on variable rate basis				0.50%
Wells Fargo Bank Group Credit Facility Tranche One [Member] | London Interbank Offered Rate (LIBOR) [Member]
Line of Credit Facility [Line Items]
Line of credit facility, spread on variable rate basis				1.00%
Wells Fargo Bank Group Credit Facility Tranche Two [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment						600,000,000			600,000,000
Wells Fargo Bank Group Credit Facility Tranche Two [Member] | Debt Instrument Redemption Period Two [Member]
Line of Credit Facility [Line Items]
Revolving credit borrowings									1,649,000
Citibank Bilateral Agreement [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment						200,000,000			200,000,000
Percentage of interest on the principal amount of letters of credit issued				0.35%
Percentage of interest on the principal amount of issued extended tenor letters of credit				0.45%
Maximum term allowed to execute issued letters of credit				60 months
Per annum commitment fee on undrawn credit				0.15%
Revolving credit borrowings						$ 146,289,000			$ 147,970,000
Lloyds Bank plc Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment | £				£ 52,175,000				£ 47,000,000
Line of credit facility, required minimum debt-to-capital ratio | Rate				35.00%		35.00%
Line of credit facility, required minimum net worth						$ 5,532,663,000
Consolidated net worth, percentage					70.00%
Percentage amount of consolidated net income required to be reserved to maintain compliance with facility covenants				25.00%
Percentage amount of increase in consolidated net worth from new issuance required to be reserved to maintain compliance with facility covenants				25.00%
Increase in reserves due to new issuance during the period required to maintain compliance with facility covenants						$ 5,910,400,000
Percentage commitment fee per annum				0.10%
Per annum commitment fee on undrawn credit				0.35%
Revolving credit borrowings | £				£ 52,175,000				£ 47,000,000