Financial Instruments - Credit Facilities (Details)	3 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity	$ 71,000,000
Revolving credit facility, minimum interest coverage ratio	3.50
Line of credit facility	$ 0	$ 0
Operational Efficiency
Line of Credit Facility [Line Items]
Aggregate amount of all charges	$ 100,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Revolving credit facility, term	5 years
Revolving credit facility, current borrowing capacity	$ 1,000,000,000.0
Line of credit facility, maximum borrowing capacity	$ 1,500,000,000.0
Revolving credit facility, covenant compliance ratio	3.50
Maximum total leverage ratio	4.00