Variable Interest Entities - Investments by Ownership Percentage with Fair Value Election (Details) - Investment funds - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Variable Interest Entity [Line Items]
Fair value option investment funds	$ 724	$ 661
3% – 49%
Variable Interest Entity [Line Items]
Fair value option investment funds	590	562
Less than 3%
Variable Interest Entity [Line Items]
Fair value option investment funds	$ 134	$ 99