Investments - Amortized Cost and Fair Value of Fixed Maturity Available-for-Sale Securities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Equity Securities, Amortized Cost Basis	$ 1,490	$ 1,392
Due in one year or less, Amortized Cost	242
Due after one year through five years, Amortized Cost	1,055
Due after five years through ten years, Amortized Cost	2,462
Due after ten years, Amortized Cost	11,195
Subtotal, Amortized Cost	14,954
Other securities which provide for periodic payments, Amortized Cost	7,189
Total fixed maturity available-for-sale securities, Amortized Cost	22,143	$ 20,847
Due in one year or less, Fair Value	241
Due after one year through five years, Fair Value	1,039
Due after five years through ten years, Fair Value	2,384
Due after ten years, Fair Value	10,601
Subtotal, Fair Value	14,265
Other securities which provide for periodic payments, Fair Value	7,156
Total fixed maturity available-for-sale securities, Fair Value	21,421
Asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	3,705
Other securities which provide for periodic payments, Fair Value	3,682
Commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	1,808
Other securities which provide for periodic payments, Fair Value	1,794
Residential mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Other securities which provide for periodic payments, Amortized Cost	1,676
Other securities which provide for periodic payments, Fair Value	$ 1,680