Debt - Revolving Credit and Letter of Credit (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) Banks
Line of Credit Facility [Line Items]
Debt outstanding	$ 5,400
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facility	$ 1,500
Expiration date	Dec. 31, 2022
Number of participating banks | Banks	15
Maximum borrowing capacity under option to increase	$ 2,000
Covenant terms	The agreement contains customary covenants and restrictions, including a financial covenant that the Company may not permit its leverage ratio – total consolidated debt to total consolidated capitalization (each as defined in the credit agreement) – to be greater than 50%.
Leverage ratio covenant	50.00%
Borrowing capacity within maximum debt coverage covenant	$ 9,300
Debt covenant compliance	The Company was in compliance with its debt covenants as of December 31, 2017.
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facility	$ 500
Letters of credit outstanding	$ 11