Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Assets:
Fixed-maturity securities	$ 11,048	$ 11,301
Other invested assets	103	94
Liabilities:
Credit derivative liabilities	237	271
Investment funds, fair value	42	45
Recurring [Member]
Assets:
Other invested assets	7	7
FG VIEs’ assets, at fair value	651	700
Other assets	124	123
Total assets carried at fair value	11,830	12,131
Liabilities:
Credit derivative liabilities	237	271
FG VIEs’ liabilities with recourse, at fair value	598	627
FG VIEs’ liabilities without recourse, at fair value	110	130
Total liabilities carried at fair value	945	1,028
Recurring [Member] \| Level 1 [Member]
Assets:
Other invested assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	29	25
Total assets carried at fair value	296	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	35	36
Total assets carried at fair value	9,548	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	651	700
Other assets	60	62
Total assets carried at fair value	1,986	2,034
Liabilities:
Credit derivative liabilities	237	271
FG VIEs’ liabilities with recourse, at fair value	598	627
FG VIEs’ liabilities without recourse, at fair value	110	130
Total liabilities carried at fair value	945	1,028
Fixed Maturities [Member]
Assets:
Fixed-maturity securities	10,297	10,674
Fixed Maturities [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,474	5,760
Fixed Maturities [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	275	285
Fixed Maturities [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	1,924	2,018
Fixed Maturities [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	849	861
Fixed Maturities [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	559	549
Fixed Maturities [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	889	896
Fixed Maturities [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	327	305
Fixed Maturities [Member] \| Recurring [Member]
Assets:
Fixed-maturity securities	10,297	10,674
Fixed Maturities [Member] \| Recurring [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,474	5,760
Fixed Maturities [Member] \| Recurring [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	275	285
Fixed Maturities [Member] \| Recurring [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	1,924	2,018
Fixed Maturities [Member] \| Recurring [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	849	861
Fixed Maturities [Member] \| Recurring [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	559	549
Fixed Maturities [Member] \| Recurring [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	889	896
Fixed Maturities [Member] \| Recurring [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	327	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	9,029	9,410
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,391	5,684
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| US government and agencies [Member]
Assets:
Fixed-maturity securities	275	285
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Corporate securities [Member]
Assets:
Fixed-maturity securities	1,862	1,951
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	535	527
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| CMBS [Member]
Assets:
Fixed-maturity securities	559	549
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	80	109
Fixed Maturities [Member] \| Recurring [Member] \| Level 2 [Member] \| Foreign government securities [Member]
Assets:
Fixed-maturity securities	327	305
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member]
Assets:
Fixed-maturity securities	1,268	1,264
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	83	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	62	67
Fixed Maturities [Member] \| Recurring [Member] \| Level 3 [Member] \| RMBS [Member]
Assets:
Fixed-maturity securities	314	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	809	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	751	627
Short-term Investments [Member] \| Recurring [Member]
Assets:
Fixed-maturity securities	751	627
Short-term Investments [Member] \| Recurring [Member] \| Level 1 [Member]
Assets:
Fixed-maturity securities	267	464
Short-term Investments [Member] \| Recurring [Member] \| Level 2 [Member]
Assets:
Fixed-maturity securities	484	162
Short-term Investments [Member] \| Recurring [Member] \| Level 3 [Member]
Assets:
Fixed-maturity securities	$ 0	$ 1

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