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		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance as of beginning of the period	$ 553	$ 385	$ 437	$ 454
Reported in Other comprehensive income (loss)	15	0	42	(7)
Total realized and unrealized investment gains (losses)	15	(1)	44	(5)
Purchases	101	122	255	195
Sales	0	0	0	(77)
Settlements	(7)	(29)	(19)	(45)
Transfers into Level 3	0	59	45	77
Transfers out of Level 3	(16)	(32)	(116)	(95)
Balance as of end of the period	646	504	646	504
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	(1)	2	(4)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	16	1	38	(4)
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance as of beginning of the period	338	94	222	98
Reported in Other comprehensive income (loss)	14	0	34	(1)
Total realized and unrealized investment gains (losses)	14	0	34	(2)
Purchases	79	67	211	69
Sales	0	0	0	(5)
Settlements	(3)	(3)	(7)	(7)
Transfers into Level 3	0	30	0	35
Transfers out of Level 3	0	0	(32)	0
Balance as of end of the period	428	188	428	188
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	14	0	29	(2)
States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance as of beginning of the period	0	1	0	1
Reported in Other comprehensive income (loss)	0	0	0	0
Total realized and unrealized investment gains (losses)	0	0	0	0
Purchases	0	0	0	0
Sales	0	0	0	0
Settlements	0	(1)	0	(1)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance as of end of the period	0	0	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	0	0	0
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	0	0	0	0
Asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance as of beginning of the period	193	273	197	335
Reported in Other comprehensive income (loss)	1	0	8	(6)
Total realized and unrealized investment gains (losses)	1	(2)	8	(1)
Purchases	22	55	42	126
Sales	0	0	0	(72)
Settlements	(4)	(25)	(12)	(37)
Transfers into Level 3	0	29	45	42
Transfers out of Level 3	(16)	(32)	(84)	(95)
Balance as of end of the period	196	298	196	298
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	(2)	0	(2)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	2	1	9	(2)
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance as of beginning of the period	22	17	18	20
Reported in Other comprehensive income (loss)	0	0	0	0
Total realized and unrealized investment gains (losses)	0	1	2	(2)
Purchases	0	0	2	0
Sales	0	0	0	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance as of end of the period	22	18	22	18
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Net income (loss) in the period	0	1	2	(2)
Unrealized gains (losses) on Level 3 assets and liabilities recognized in Other comprehensive income (loss) in the period	0	0	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	(1)	2	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	0	(1)
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	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	(2)	0	5
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	$ 1	$ 2	$ (2)