Long-Term Debt and Credit Facilities - Summary of Credit Facilities (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2014
Line of credit facility [Line Items]
Unused borrowing capacity		$ 2,949
Domestic Revolving Credit Facility 1850 Million Capacity
Line of credit facility [Line Items]
Notes, term	5 years
Maximum borrowing capacity	$ 1,850
Amended Domestic Revolving Credit Facility 2370 Million Capacity
Line of credit facility [Line Items]
Maximum borrowing capacity				$ 2,370
Domestic Revolving Credit Facility 165 Million Capacity
Line of credit facility [Line Items]
Maximum borrowing capacity			$ 165