Credit Facilities (Narrative) (Details)			3 Months Ended	12 Months Ended
	May 26, 2016 USD ($)	Jun. 22, 2012 USD ($)	Mar. 31, 2021 GBP (£) Item Rate	Dec. 31, 2019	Mar. 31, 2021 USD ($) Item Rate	Dec. 31, 2020 GBP (£)	Dec. 31, 2020 USD ($)	Mar. 25, 2020 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, commitment			£ 52,175,000		$ 1,050,000,000
Maximum percentage of admitted assets allowed for borrowing base			10.00%		10.00%
Advances from FHLB					$ 310,000,000		$ 310,000,000
Wells Fargo Bank Group Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, term of credit facility	5 years	4 years
Line of credit facility, commitment	$ 800,000,000	$ 800,000,000			$ 800,000,000		800,000,000
Line of credit facility, number of tranches | Item			2		2
Line of credit facility, required 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					6,476,618,000
Revolving credit borrowings					749,766,000		753,932,000
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
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 Bilateral LOC Agreement [Member]
Line of Credit Facility [Line Items]
Line of credit facility, expiration date			Dec. 31, 2021
Line of credit facility, commitment					50,000,000
Revolving credit borrowings					42,893,000
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					$ 186,277,000		$ 185,483,000
Citibank Bilateral Agreement [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Line of credit facility, term of credit facility			15 months
Lloyds Bank plc Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of credit facility, commitment | £			£ 52,175,000			£ 52,175,000
Line of credit facility, required debt-to-capital ratio | Rate			35.00%		35.00%
Line of credit facility, required minimum net worth					$ 6,393,047,000
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					6,613,571,000
Percentage commitment fee per annum			0.10%
Per annum commitment fee on undrawn credit			0.35%
Revolving credit borrowings | £			£ 52,175,000			£ 52,175,000
Federal Home Loan Bank Of New York [Member]
Line of Credit Facility [Line Items]
Admitted assets for FHLB					17,280,068,000
Maximum amount available under FHLB					1,728,007,000
Advances from FHLB					400,000,000
Revolving credit borrowings					$ 310,000
FHLB advance interest rate			0.35%		0.35%
Amount of borrowed funds required to be used to acquire additional membership stock, percentage			4.50%
Federal Home Loan Bank Of New York [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
FHLB advance maturity			Nov. 30, 2021
Federal Home Loan Bank Of New York [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
FHLB advance maturity			Dec. 31, 2021