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, 2020	Mar. 31, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	$ 651	$ 437
Reported in Other comprehensive income (loss)	(46)	11
Total realized and unrealized investment gains (losses)	(49)	13
Purchases	112	76
Sales	0	0
Settlements	(5)	(6)
Transfers into Level 3	0	5
Transfers out of Level 3	(1)	(68)
Balance, Ending, Assets	708	457
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Net income (loss) in the period	(3)	2
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Other comprehensive income (loss) in the period	(44)	10
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	468	222
Reported in Other comprehensive income (loss)	(37)	8
Total realized and unrealized investment gains (losses)	(37)	8
Purchases	67	56
Sales	0	0
Settlements	(2)	(2)
Transfers into Level 3	0	0
Transfers out of Level 3	0	(31)
Balance, Ending, Assets	496	253
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Other comprehensive income (loss) in the period	(35)	7
States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	0	0
Reported in Other comprehensive income (loss)	0	0
Total realized and unrealized investment gains (losses)	0	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	0	0
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Other comprehensive income (loss) in the period	0	0
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	165	197
Reported in Other comprehensive income (loss)	(9)	3
Total realized and unrealized investment gains (losses)	(9)	3
Purchases	45	20
Sales	0	0
Settlements	(3)	(4)
Transfers into Level 3	0	5
Transfers out of Level 3	(1)	(37)
Balance, Ending, Assets	197	184
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Net income (loss) in the period	0	0
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Other comprehensive income (loss) in the period	(9)	3
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, Beginning, Assets	18	18
Reported in Other comprehensive income (loss)	0	0
Total realized and unrealized investment gains (losses)	(3)	2
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	15	20
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 recognized in Net income (loss) in the period	(3)	2
Unrealized gains (losses) on Level 3 assets and liabilities held as of March 31, 2020 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)	0	2
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)	0	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)	0	2
Reported in Net investment income
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Reported in Net investment income	(3)	0
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	0	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	$ (3)	$ 0