Debt - Credit Facility (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Line of Credit Facility, Fair Value of Amount Outstanding		$ 0	$ 0
Letters of Credit Outstanding, Amount		0	0
Commercial Paper and Credit Facility Maximum Borrowing Capacity		1,800,000,000.0
Commercial Paper		$ 0	$ 0
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Short-term Debt, Commercial Paper Maximum Maturity Days		390 days
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Accordion Feature	$ 750,000,000
Line of Credit Facility, Annual Fee Percentage		0.20%
Revolving Credit Facility [Member] | Minimum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate, Base Rate	0.00%
Line of Credit Facility, Interest Rate, LIBOR Plus All-in Spread Rate	1.00%
All-in Interest Rate Spread Based on Credit Rating		0.175%
Line of Credit Facility, Annual Fee Percentage	0.125%
Line of Credit Facility, Covenant Terms	3.00x
Revolving Credit Facility [Member] | Maximum [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate, Base Rate	0.70%
Line of Credit Facility, Interest Rate, LIBOR Plus All-in Spread Rate	1.70%
All-in Interest Rate Spread Based on Credit Rating		1.175%
Line of Credit Facility, Annual Fee Percentage	0.30%
Line of Credit Facility, Covenant Terms	4.25x
Revolving Credit Facility [Member] | Previous Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Former Borrowing Capacity	$ 2,000,000,000
Line of Credit Facility, Expiration Date	Mar. 31, 2019
Letter of credit, credit facility sub-facility, maximum borrowing amount	$ 300,000,000
Debt Issuance Costs, Gross	2,000,000
Revolving Credit Facility [Member] | Amended and Restated Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 1,800,000,000
Line of Credit Facility, Expiration Date	Aug. 31, 2022
Letter of credit, credit facility sub-facility, maximum borrowing amount	$ 250,000,000
Debt Issuance Costs, Gross	$ 5,000,000
Commercial Paper [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 2,000,000,000.0