Fair Value Measurement - Financial Instruments Carried at Fair Value (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets:
Fixed-maturity securities	$ 10,716	$ 10,818
Other invested assets	61	55
Liabilities:
Credit derivative liabilities	229	209
Recurring [Member]
Assets:
Other invested assets	7	7
FG VIEs’ assets, at fair value	560	569
Other assets	137	139
Total assets carried at fair value	11,420	11,533
Liabilities:
Credit derivative liabilities	229	209
FG VIEs’ liabilities with recourse, at fair value	505	517
FG VIEs’ liabilities without recourse, at fair value	104	102
Total liabilities carried at fair value	838	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	484	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	41	38
Total assets carried at fair value	8,875	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	7	7
FG VIEs’ assets, at fair value	560	569
Other assets	66	76
Total assets carried at fair value	2,061	2,063
Liabilities:
Credit derivative liabilities	229	209
FG VIEs’ liabilities with recourse, at fair value	505	517
FG VIEs’ liabilities without recourse, at fair value	104	102
Total liabilities carried at fair value	838	828
Fixed Maturities [Member]
Assets:
Fixed-maturity securities	9,989	10,089
Fixed Maturities [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,843	4,911
Fixed Maturities [Member] | US government and agencies [Member]
Assets:
Fixed-maturity securities	166	175
Fixed Maturities [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,154	2,136
Fixed Maturities [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	959	982
Fixed Maturities [Member] | CMBS [Member]
Assets:
Fixed-maturity securities	539	539
Fixed Maturities [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	1,076	1,068
Fixed Maturities [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	252	278
Fixed Maturities [Member] | Recurring [Member]
Assets:
Fixed-maturity securities	9,989	10,089
Fixed Maturities [Member] | Recurring [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,843	4,911
Fixed Maturities [Member] | Recurring [Member] | US government and agencies [Member]
Assets:
Fixed-maturity securities	166	175
Fixed Maturities [Member] | Recurring [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,154	2,136
Fixed Maturities [Member] | Recurring [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	959	982
Fixed Maturities [Member] | Recurring [Member] | CMBS [Member]
Assets:
Fixed-maturity securities	539	539
Fixed Maturities [Member] | Recurring [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	1,076	1,068
Fixed Maturities [Member] | Recurring [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	252	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	8,561	8,678
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	4,739	4,812
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | US government and agencies [Member]
Assets:
Fixed-maturity securities	166	175
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Corporate securities [Member]
Assets:
Fixed-maturity securities	2,106	2,080
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	641	673
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | CMBS [Member]
Assets:
Fixed-maturity securities	539	539
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Asset-backed Securities [Member]
Assets:
Fixed-maturity securities	118	121
Fixed Maturities [Member] | Recurring [Member] | Level 2 [Member] | Foreign government securities [Member]
Assets:
Fixed-maturity securities	252	278
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Fixed-maturity securities	1,428	1,411
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | Obligations of state and political subdivisions [Member]
Assets:
Fixed-maturity securities	104	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	48	56
Fixed Maturities [Member] | Recurring [Member] | Level 3 [Member] | RMBS [Member]
Assets:
Fixed-maturity securities	318	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	958	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	727	729
Short-term Investments [Member] | Recurring [Member]
Assets:
Fixed-maturity securities	727	729
Short-term Investments [Member] | Recurring [Member] | Level 1 [Member]
Assets:
Fixed-maturity securities	454	429
Short-term Investments [Member] | Recurring [Member] | Level 2 [Member]
Assets:
Fixed-maturity securities	273	300
Short-term Investments [Member] | Recurring [Member] | Level 3 [Member]
Assets:
Fixed-maturity securities	$ 0	$ 0