Investments - Equity Securities (Details) $ in Millions	Mar. 31, 2018 USD ($)
Investments:
Fair value of equity securities that have a readily determinable fair value	$ 454
Amortized cost of equity securities that have a readily determinable fair value	456
Carrying value of private equity securities that do not have a readily determinable fair value	66
Impairment or value changes of equity securities without a readily determinable fair value