Investments - Debt Maturities (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Fixed maturities
Available-for-sale Securities Including Securities Pledged [Line Items]
One year or less, Amortized Cost	$ 464
One year or less, Fair Value	468
After one year through five years, Amortized Cost	4,244
After one year through five years, Fair Value	4,311
After five years through ten years, Amortized Cost	5,901
After five years through ten years, Fair Value	5,863
After ten years, Amortized Cost	8,013
After ten years, Fair Value	8,364
Fixed maturities, including securities pledged, Amortized Cost	24,214	$ 23,579
Fixed maturities, including securities pledged, Fair Value	24,650	$ 25,042
Mortgage-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	4,508
Without single maturity date, Fair Value	$ 4,559
Percentage collateralized of mortgage backed securities including interest-only strip or principal-only strip	53.80%	52.10%
Other asset-backed securities
Available-for-sale Securities Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	$ 1,084
Without single maturity date, Fair Value	1,085
Fixed maturities, including securities pledged, Amortized Cost	1,084	$ 671
Fixed maturities, including securities pledged, Fair Value	$ 1,085	$ 681