Investments - Debt Maturities (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 541
One year or less, Fair Value	547
After one year through five years, Amortized Cost	4,012
After one year through five years, Fair Value	4,057
After five years through ten years, Amortized Cost	5,937
After five years through ten years, Fair Value	5,889
After ten years, Amortized Cost	8,142
After ten years, Fair Value	8,445
Fixed maturities, including securities pledged, Amortized Cost	24,341	$ 23,579
Fixed maturities, including securities pledged, Fair Value	24,661	$ 25,042
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	4,568
Without single maturity date, Fair Value	$ 4,584
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	52.30%	52.10%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	$ 1,141
Without single maturity date, Fair Value	1,139
Fixed maturities, including securities pledged, Amortized Cost	1,141	$ 671
Fixed maturities, including securities pledged, Fair Value	$ 1,139	$ 681