Investments (excluding Consolidated Investment Entities) - Debt Maturities (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Including Securities Pledged [Line Items]
Fixed maturities, including securities pledged, Amortized Cost Basis	$ 44,366	$ 44,743
Fixed maturities, including securities pledged	48,329	47,394
Fixed maturities
Available-for-sale Securities, Including Securities Pledged [Line Items]
One year or less, Amortized Cost	988
One year or less, Fair Value	1,001
After one year through five years, Amortized Cost	8,389
After one year through five years, Fair Value	8,703
After five years through ten years, Amortized Cost	10,352
After five years through ten years, Fair Value	10,762
After ten years, Amortized Cost	21,097
After ten years, Fair Value	24,212
Fixed maturities, including securities pledged, Amortized Cost Basis	49,207	49,069
Fixed maturities, including securities pledged	53,434	51,868
Mortgage-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	6,893
Without single maturity date, Fair Value	7,228
Other asset-backed securities
Available-for-sale Securities, Including Securities Pledged [Line Items]
Without single maturity date, Amortized Cost	1,488
Without single maturity date, Fair Value	1,528
Fixed maturities, including securities pledged, Amortized Cost Basis	1,488	1,096
Fixed maturities, including securities pledged	$ 1,528	$ 1,134