Fair Value Measurements - Fair Values of Separate Account Assets (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guaranteed separate accounts	$ 513	$ 523
Non-guaranteed separate accounts	7,790	7,880
Subtotal	8,303	8,403
Non-guaranteed separate accounts priced at NAV as a practical expedient	847	683
Separate account assets per Consolidated Balance Sheets	9,150	9,086
Pension Plan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-guaranteed separate accounts	4,400	4,200
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guaranteed separate accounts	223	226
Non-guaranteed separate accounts	1,982	1,925
Subtotal	2,205	2,151
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guaranteed separate accounts	290	297
Non-guaranteed separate accounts	5,463	5,600
Subtotal	5,753	5,897
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Guaranteed separate accounts	0	0
Non-guaranteed separate accounts	345	355
Subtotal	345	355
Significant Unobservable Inputs (Level 3) | Pension Plan
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Non-guaranteed separate accounts	$ 300	$ 300