Short-Term Debt - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short Term Debt [Line Items]
Short-term debt	$ 849,000,000	$ 628,000,000
Proceeds from short-term borrowings with maturities greater than three months		60,000,000	$ 35,000,000
Maturity in More Than Three Months
Short Term Debt [Line Items]
Proceeds from short-term borrowings with maturities greater than three months	0
Commercial Paper Program
Short Term Debt [Line Items]
Short-term debt	$ 849,000,000	$ 628,000,000