Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets:
Fixed-maturity securities	$ 10,419	$ 10,818
Other invested assets	57	55
Liabilities:
Credit derivative liabilities	214	209
Recurring [Member]
Assets:
Other invested assets	6	7
FG VIEs’ assets, at fair value	469	569
Other assets	148	139
Total assets carried at fair value	11,042	11,533
Liabilities:
Credit derivative liabilities	214	209
FG VIEs’ liabilities with recourse, at fair value	388	517
FG VIEs’ liabilities without recourse, at fair value	105	102
Total liabilities carried at fair value	707	828
Recurring [Member] | Level 1 [Member]
Assets:
Other invested assets	0	0
FG VIEs’ assets, at fair value	0	0
Other assets	30	25
Total assets carried at fair value	918	454
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	38
Total assets carried at fair value	8,438	9,016
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	6	7
FG VIEs’ assets, at fair value	469	569
Other assets	76	76
Total assets carried at fair value	1,686	2,063
Liabilities:
Credit derivative liabilities	214	209
FG VIEs’ liabilities with recourse, at fair value	388	517
FG VIEs’ liabilities without recourse, at fair value	105	102
Total liabilities carried at fair value	707	828
Fixed Maturities [Member]
Assets:
Fixed-maturity securities	9,277	10,089
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,463	4,911
Fixed Maturities [Member] | US government and agencies [Member]
Assets:
Fixed-maturity securities	155	175
Fixed Maturities [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,339	2,136
Fixed Maturities [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	877	982
Fixed Maturities [Member] | CMBS [Member]
Assets:
Fixed-maturity securities	469	539
Fixed Maturities [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	775	1,068
Fixed Maturities [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	199	278
Fixed Maturities [Member] | Recurring [Member]
Assets:
Fixed-maturity securities	9,277	10,089
Fixed Maturities [Member] | Recurring [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,463	4,911
Fixed Maturities [Member] | Recurring [Member] | US government and agencies [Member]
Assets:
Fixed-maturity securities	155	175
Fixed Maturities [Member] | Recurring [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,339	2,136
Fixed Maturities [Member] | Recurring [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	877	982
Fixed Maturities [Member] | Recurring [Member] | CMBS [Member]
Assets:
Fixed-maturity securities	469	539
Fixed Maturities [Member] | Recurring [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	775	1,068
Fixed Maturities [Member] | Recurring [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	199	278
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	8,142	8,678
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,354	4,812
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | US government and agencies [Member]
Assets:
Fixed-maturity securities	155	175
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,293	2,080
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	561	673
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | CMBS [Member]
Assets:
Fixed-maturity securities	469	539
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	111	121
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	199	278
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Fixed-maturity securities	1,135	1,411
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	109	99
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	46	56
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	316	309
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	664	947
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	1,142	729
Short-term Investments [Member] | Recurring [Member]
Assets:
Fixed-maturity securities	1,142	729
Short-term Investments [Member] | Recurring [Member] | Level 1 [Member]
Assets:
Fixed-maturity securities	888	429
Short-term Investments [Member] | Recurring [Member] | Level 2 [Member]
Assets:
Fixed-maturity securities	254	300
Short-term Investments [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Fixed-maturity securities	$ 0	$ 0