Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Assets:
Fixed-maturity securities	$ 11,301	$ 10,823
Other invested assets	94	162
Credit derivative assets	2	13
Liabilities:
Credit derivative liabilities	271	402
Investment funds measured using Net Asset Value excluded from fair value hierarchy	45	48
Recurring [Member]
Assets:
Other invested assets	7	8
Credit derivative assets	2	13
FG VIEs’ assets, at fair value	700	876
Other assets	121	114
Total assets carried at fair value	12,131	11,834
Liabilities:
Credit derivative liabilities	271	402
FG VIEs’ liabilities with recourse, at fair value	627	807
FG VIEs’ liabilities without recourse, at fair value	130	151
Total liabilities carried at fair value	1,028	1,360
Recurring [Member] | Level 1 [Member]
Assets:
Other invested assets	0	0
Credit derivative assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	25	24
Total assets carried at fair value	489	343
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
Credit derivative assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	36	28
Total assets carried at fair value	9,608	9,263
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	8
Credit derivative assets	2	13
FG VIEs’ assets, at fair value	700	876
Other assets	60	62
Total assets carried at fair value	2,034	2,228
Liabilities:
Credit derivative liabilities	271	402
FG VIEs’ liabilities with recourse, at fair value	627	807
FG VIEs’ liabilities without recourse, at fair value	130	151
Total liabilities carried at fair value	1,028	1,360
Fixed Maturities [Member]
Assets:
Fixed-maturity securities	10,674	10,233
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,760	5,432
Fixed Maturities [Member] | US Treasury and Government [Member]
Assets:
Fixed-maturity securities	285	440
Fixed Maturities [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,018	1,613
Fixed Maturities [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Assets:
Fixed-maturity securities	861	987
Fixed Maturities [Member] | Commercial Mortgage Backed Securities (CMBS) [Member]
Assets:
Fixed-maturity securities	549	583
Fixed Maturities [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	896	945
Fixed Maturities [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	305	233
Fixed Maturities [Member] | Recurring [Member]
Assets:
Fixed-maturity securities	10,674	10,233
Fixed Maturities [Member] | Recurring [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,760	5,432
Fixed Maturities [Member] | Recurring [Member] | US Treasury and Government [Member]
Assets:
Fixed-maturity securities	285	440
Fixed Maturities [Member] | Recurring [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,018	1,613
Fixed Maturities [Member] | Recurring [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Assets:
Fixed-maturity securities	861	987
Fixed Maturities [Member] | Recurring [Member] | Commercial Mortgage Backed Securities (CMBS) [Member]
Assets:
Fixed-maturity securities	549	583
Fixed Maturities [Member] | Recurring [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	896	945
Fixed Maturities [Member] | Recurring [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	305	233
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 Treasury and Government [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] | Residential Mortgage-Backed Securities (RMBS) [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] | Recurring [Member] | Level 1 [Member] | Commercial Mortgage Backed Securities (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	9,410	8,964
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	5,684	5,393
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | US Treasury and Government [Member]
Assets:
Fixed-maturity securities	285	440
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	1,951	1,553
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Assets:
Fixed-maturity securities	527	622
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Commercial Mortgage Backed Securities (CMBS) [Member]
Assets:
Fixed-maturity securities	549	583
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	109	140
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	305	233
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Fixed-maturity securities	1,264	1,269
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	76	39
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | US Treasury and Government [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	67	60
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | Residential Mortgage-Backed Securities (RMBS) [Member]
Assets:
Fixed-maturity securities	334	365
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | Commercial Mortgage Backed Securities (CMBS) [Member]
Assets:
Fixed-maturity securities	0	0
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	787	805
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	627	590
Short-term Investments [Member] | Recurring [Member]
Assets:
Fixed-maturity securities	627	590
Short-term Investments [Member] | Recurring [Member] | Level 1 [Member]
Assets:
Fixed-maturity securities	464	319
Short-term Investments [Member] | Recurring [Member] | Level 2 [Member]
Assets:
Fixed-maturity securities	162	271
Short-term Investments [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Fixed-maturity securities	$ 1	$ 0