Available-for-Sale Securities - Additional Information (Details)	6 Months Ended
	Jun. 30, 2017 USD ($) position	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($) position
Investments, Debt and Equity Securities [Abstract]
Cash excluded from available-for-sale debt securities table	$ 1,800,000,000		$ 2,500,000,000
Securities in unrealized loss positions, number of positions (securities) | position	2,303		2,709
Other than temporary impairment losses, investments	$ 0	$ 0