Fair Value - 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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	$ 339	$ 520	$ 407	$ 473
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	0	9	12
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	2	7	8	14
Purchases	52	78	109	279
Sales	(1)	0	(64)	(61)
Settlements	(24)	(15)	(67)	(61)
Transfers into Level 3	116	0	168	5
Transfers out of Level 3	0	(96)	(85)	(167)
Balance, end of year	485	494	485	494
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	(2)	0	0
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	100	242	130	168
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	1	1	1
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	7	2	14
Purchases	13	16	18	163
Sales	0	0	(1)	(36)
Settlements	(11)	(5)	(36)	(15)
Transfers into Level 3	15	0	15	0
Transfers out of Level 3	0	0	(10)	(34)
Balance, end of year	119	261	119	261
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	0	0	0
States, municipalities and political subdivisions
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	1	2	1	2
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	0	0	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	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, end of year	1	1	1	1
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	0	0	0
Residential mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	123	134	129	134
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	0	3	2
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	(1)	4	(2)
Purchases	0	5	0	15
Sales	0	0	0	0
Settlements	(7)	(1)	(18)	(10)
Transfers into Level 3	58	0	58	0
Transfers out of Level 3	0	(58)	0	(60)
Balance, end of year	176	79	176	79
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	0	0	0
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	13	11	13	22
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	0	0	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	(1)	0	(1)	0
Purchases	12	23	12	32
Sales	0	0	0	0
Settlements	(2)	(8)	(2)	(17)
Transfers into Level 3	2	0	2	3
Transfers out of Level 3	0	(2)	0	(16)
Balance, end of year	24	24	24	24
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	0	0	0
Other asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	82	45	57	53
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	(1)	0	(2)	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	0	1	2
Purchases	27	34	78	69
Sales	0	0	0	(25)
Settlements	(4)	0	(6)	(1)
Transfers into Level 3	41	0	93	2
Transfers out of Level 3	0	(36)	(75)	(57)
Balance, end of year	146	43	146	43
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	0	0	0
Total asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	218	190	199	209
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	0	1	2
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	(1)	4	0
Purchases	39	62	90	116
Sales	0	0	0	(25)
Settlements	(13)	(9)	(26)	(28)
Transfers into Level 3	101	0	153	5
Transfers out of Level 3	0	(96)	(75)	(133)
Balance, end of year	346	146	346	146
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	0	0	0
Total fixed maturity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	319	434	330	379
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	1	2	3
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	2	6	6	14
Purchases	52	78	108	279
Sales	0	0	(1)	(61)
Settlements	(24)	(15)	(62)	(44)
Transfers into Level 3	116	0	168	5
Transfers out of Level 3	0	(96)	(85)	(167)
Balance, end of year	466	408	466	408
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	0	0	0
Equity securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	19	19	19	20
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	(1)	0	(1)
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0	1	2	0
Purchases	0	0	1	0
Sales	0	0	(3)	0
Settlements	0	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, end of year	19	19	19	19
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	(2)	0	(2)
Derivative financial instruments
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year			0
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)			1
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)			0
Purchases			0
Sales			(1)
Settlements			0
Transfers into Level 3			0
Transfers out of Level 3			0
Balance, end of year	0		0
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance			0
Life settlement contracts, included in Other assets
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	1	67	58	74
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	0	6	10
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Sales	(1)	0	(59)	0
Settlements	0	0	(5)	(17)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, end of year	0	67	0	67
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	$ 0	$ 0	$ 0	$ 2