Fair Value (Table of reconciliation for assets and liabilities measured at fair value on a recurring basis using significant unobservable inputs) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	$ 1,151	$ 651
Reported in Other comprehensive income (loss)	(51)	(46)
Total realized and unrealized investment gains (losses)	(60)	(49)
Purchases	72	112
Sales	0	0
Settlements	(19)	(5)
Transfers into Level 3	19	0
Transfers out of Level 3	(8)	(1)
Balance, Ending, Assets	1,155	708
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	2	(3)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(51)	(44)
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	770	468
Reported in Other comprehensive income (loss)	(40)	(37)
Total realized and unrealized investment gains (losses)	(53)	(37)
Purchases	42	67
Sales	0	0
Settlements	(2)	(2)
Transfers into Level 3	10	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	767	496
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(40)	(35)
States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	46	0
Reported in Other comprehensive income (loss)	(2)	0
Total realized and unrealized investment gains (losses)	(2)	0
Purchases	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	44	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(2)	0
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	308	165
Reported in Other comprehensive income (loss)	(9)	(9)
Total realized and unrealized investment gains (losses)	(7)	(9)
Purchases	30	45
Sales	0	0
Settlements	(17)	(3)
Transfers into Level 3	9	0
Transfers out of Level 3	(8)	(1)
Balance, Ending, Assets	315	197
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	(9)	(9)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	27	18
Reported in Other comprehensive income (loss)	0	0
Total realized and unrealized investment gains (losses)	2	(3)
Purchases	0	0
Sales	0	0
Settlements	0	0
Transfers into Level 3	0	0
Transfers out of Level 3	0	0
Balance, Ending, Assets	29	15
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	2	(3)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	0	0
Reported in Net investment gains (losses)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment gains (losses)	(12)	0
Reported in Net investment gains (losses) | Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment gains (losses)	(13)	0
Reported in Net investment gains (losses) | States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment gains (losses)	0	0
Reported in Net investment gains (losses) | Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment gains (losses)	0	0
Reported in Net investment gains (losses) | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment gains (losses)	1	0
Reported in Net investment income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	3	(3)
Reported in Net investment income | Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	0	0
Reported in Net investment income | States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	0	0
Reported in Net investment income | Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	2	0
Reported in Net investment income | Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	$ 1	$ (3)