Debt - Credit Facility (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
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
Revolving Credit Facility [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Accordion Feature	750,000,000
Debt guaranteed by domestic subsidiary	$ 100,000,000
Line of Credit Facility, Annual Fee Percentage	0.25%
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	0.375%
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	1.375%
Line of Credit Facility, Annual Fee Percentage	0.30%
Line of Credit Facility, Covenant Terms	4.25x
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
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