Debt - Revolving Credit and Letter of Credit (Details) $ in Millions	9 Months Ended
	Sep. 30, 2017 USD ($) Banks
Line of Credit Facility [Line Items]
Debt outstanding	$ 5,300
Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Credit agreement term	5 years
Maximum borrowing capacity under credit facility	$ 1,500
Expiration date	Dec. 12, 2019
Number of participating banks | Banks	16
Maximum borrowing capacity under option to increase	$ 2,000
Leverage ratio covenant	50.00%
Borrowing capacity within maximum debt coverage covenant	$ 9,800
Letter of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity under credit facility	500
Letters of credit outstanding	$ 11
Primary Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	3
Percent of commitment held by each primary commitment holder	12.00%
Minor Commitment Holders [Member] | Revolving Credit And Letter Of Credit Facility [Member]
Line of Credit Facility [Line Items]
Number of participating banks | Banks	13
Collective percentage held by remaining commitment holders	64.00%