Investments - Mortgage Loans, Debt Service Coverage Ratio (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Loans and Leases Receivable Disclosure [Line Items]
Mortgage loans, net of allowances	$ 14,591	$ 14,306
Commercial mortgage loans
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	1,496
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,567
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	131
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,047
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,750
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	4,399
Mortgage loans, net of allowances	11,158	10,505
Mortgage loans, before allowance for credit losses	11,390	10,515
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	1,414
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,567
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	131
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	1,047
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,657
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	4,378
Mortgage loans, net of allowances		10,412
Mortgage loans, before allowance for credit losses	11,194	10,422
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | Greater than 1.20x
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	1,039
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	1,494
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	130
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	993
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	2,657
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	3,413
Mortgage loans, net of allowances		9,212
Mortgage loans, before allowance for credit losses	9,726
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | 1.00x – 1.20x
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	375
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	66
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	1
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	31
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	965
Mortgage loans, net of allowances		1,166
Mortgage loans, before allowance for credit losses	1,438
Commercial mortgage loans | Commercial mortgage loans, excluding loans under development | Less than 1.00x
Loans and Leases Receivable Disclosure [Line Items]
Financing Receivable, Year One, Originated, Current Fiscal Year	0
Financing Receivable, Originated, More than Five Years before Current Fiscal Year	7
Financing Receivable, Year Five, Originated, Four Years before Current Fiscal Year	0
Financing Receivable, Year Four, Originated, Three Years before Current Fiscal Year	23
Financing Receivable, Year Three, Originated, Two Years before Current Fiscal Year	0
Financing Receivable, Year Two, Originated, Fiscal Year before Current Fiscal Year	0
Mortgage loans, net of allowances		$ 34
Mortgage loans, before allowance for credit losses	$ 30