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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	$ 443	$ 491	$ 407	$ 473
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	8	8	12
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	2	4	6	7
Purchases	13	139	57	201
Sales	(46)	(45)	(63)	(61)
Settlements	(18)	(32)	(43)	(46)
Transfers into Level 3	24	3	52	5
Transfers out of Level 3	(80)	(48)	(85)	(71)
Balance, end of year	339	520	339	520
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	0	(3)	0	(3)
Corporate and other bonds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of year	121	193	130	168
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	1	0	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0	3	1	7
Purchases	0	94	5	147
Sales	0	(20)	(1)	(36)
Settlements	(11)	(7)	(25)	(10)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	(10)	(22)	(10)	(34)
Balance, end of year	100	242	100	242
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	0	0	0
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, end of year	1	2	1	2
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	126	128	129	134
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	1	2	2
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	(1)	3	(1)
Purchases	0	10	0	10
Sales	0	0	0	0
Settlements	(5)	(4)	(11)	(9)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	(2)
Balance, end of year	123	134	123	134
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	27	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)	0	0	0	0
Purchases	0	0	0	9
Sales	0	0	0	0
Settlements	0	(9)	0	(9)
Transfers into Level 3	0	3	0	3
Transfers out of Level 3	0	(10)	0	(14)
Balance, end of year	13	11	13	11
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	117	50	57	53
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	0	(1)	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0	2	0	2
Purchases	13	35	51	35
Sales	0	(25)	0	(25)
Settlements	(2)	(1)	(2)	(1)
Transfers into Level 3	24	0	52	2
Transfers out of Level 3	(70)	(16)	(75)	(21)
Balance, end of year	82	45	82	45
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	256	205	199	209
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	1	1	2
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	1	3	1
Purchases	13	45	51	54
Sales	0	(25)	0	(25)
Settlements	(7)	(14)	(13)	(19)
Transfers into Level 3	24	3	52	5
Transfers out of Level 3	(70)	(26)	(75)	(37)
Balance, end of year	218	190	218	190
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	378	400	330	379
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	1	2	1	2
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	4	4	8
Purchases	13	139	56	201
Sales	0	(45)	(1)	(61)
Settlements	(18)	(21)	(38)	(29)
Transfers into Level 3	24	3	52	5
Transfers out of Level 3	(80)	(48)	(85)	(71)
Balance, end of year	319	434	319	434
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	0	0	0
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	1	0	2	(1)
Purchases	0	0	1	0
Sales	(1)	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	0	0	0
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	46	72	58	74
Net realized investment gains (losses) and net change in unrealized appreciation (depreciation) included in net income (loss)	0	6	6	10
Net change in unrealized appreciation (depreciation) included in Other comprehensive income (loss)	0	0	0	0
Purchases	0	0	0	0
Sales	(45)	0	(58)	0
Settlements	0	(11)	(5)	(17)
Transfers into Level 3	0	0	0	0
Transfers out of Level 3	0	0	0	0
Balance, end of year	1	67	1	67
Unrealized gains (losses) on Level 3 assets and liabilities, ending balance	$ 0	$ (3)	$ 0	$ (3)