Long-Term Debt and Credit Facilities - Summary of Credit Facilities (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Line of credit facility [Line Items]
Unused borrowing capacity	$ 3,023
Amended Domestic Revolving Credit Facility 2370 Million Capacity
Line of credit facility [Line Items]
Notes, term	5 years
Amended Domestic Revolving Credit Facility 2650 Million Capacity
Line of credit facility [Line Items]
Notes, term	5 years
Maximum borrowing capacity	$ 2,650