Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Assets:
Fixed-maturity securities	$ 10,930	$ 11,301
Other invested assets	95	94
Liabilities:
Credit derivative liabilities	239	271
Fair Value Measured at Net Asset Value Per Share [Member]
Liabilities:
Investment funds, fair value	41	45
Recurring [Member]
Assets:
Other invested assets	7	7
FG VIEs’ assets, at fair value	596	700
Other assets	127	123
Total assets carried at fair value	11,660	12,131
Liabilities:
Credit derivative liabilities	239	271
FG VIEs’ liabilities with recourse, at fair value	545	627
FG VIEs’ liabilities without recourse, at fair value	104	130
Total liabilities carried at fair value	888	1,028
Recurring [Member] \| Level 1 [Member]
Assets:
Other invested assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	26	25
Total assets carried at fair value	486	489
Liabilities:
Credit derivative liabilities	0	0
FG VIEs’ liabilities with recourse, at fair value	0	0
FG VIEs’ liabilities without recourse, at fair value	0	0
Total liabilities carried at fair value	0	0
Recurring [Member] \| Level 2 [Member]
Assets:
Other invested assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	42	36
Total assets carried at fair value	9,101	9,608
Liabilities:
Credit derivative liabilities	0	0
FG VIEs’ liabilities with recourse, at fair value	0	0
FG VIEs’ liabilities without recourse, at fair value	0	0
Total liabilities carried at fair value	0	0
Recurring [Member] \| Level 3 [Member]
Assets:
Other invested assets	7	7
FG VIEs’ assets, at fair value	596	700
Other assets	59	62
Total assets carried at fair value	2,073	2,034
Liabilities:
Credit derivative liabilities	239	271
FG VIEs’ liabilities with recourse, at fair value	545	627
FG VIEs’ liabilities without recourse, at fair value	104	130
Total liabilities carried at fair value	888	1,028
Fixed Maturities [Member]
Assets:
Fixed-maturity securities	10,192	10,674
Fixed Maturities [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,988	5,760
Fixed Maturities [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	242	285
Fixed Maturities [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	2,147	2,018
Fixed Maturities [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	948	861
Fixed Maturities [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	528	549
Fixed Maturities [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	1,055	896
Fixed Maturities [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	284	305
Fixed Maturities [Member] \| Recurring [Member]
Assets:
Fixed-maturity securities	10,192	10,674
Fixed Maturities [Member] \| Recurring [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,988	5,760
Fixed Maturities [Member] \| Recurring [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	242	285
Fixed Maturities [Member] \| Recurring [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	2,147	2,018
Fixed Maturities [Member] \| Recurring [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	948	861
Fixed Maturities [Member] \| Recurring [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	528	549
Fixed Maturities [Member] \| Recurring [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	1,055	896
Fixed Maturities [Member] \| Recurring [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	284	305
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 1 [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member]
Assets:
Fixed-maturity securities	8,781	9,410
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,889	5,684
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	242	285
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	2,087	1,951
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	649	527
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	528	549
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	102	109
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	284	305
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member]
Assets:
Fixed-maturity securities	1,411	1,264
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	99	76
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	60	67
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	299	334
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	953	787
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	0	0
Short-term Investments [Member]
Assets:
Fixed-maturity securities	738	627
Short-term Investments [Member] \| Recurring [Member]
Assets:
Fixed-maturity securities	738	627
Short-term Investments [Member] \| Recurring [Member] \| Level 1 [Member]
Assets:
Fixed-maturity securities	460	464
Short-term Investments [Member] \| Recurring [Member] \| Level 2 [Member]
Assets:
Fixed-maturity securities	278	162
Short-term Investments [Member] \| Recurring [Member] \| Level 3 [Member]
Assets:
Fixed-maturity securities	$ 0	$ 1

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