Long-Term Debt and Credit Facilities (Details 2) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Line of credit facility [Line Items]
Unused borrowing capacity	$ 2,976
Domestic Revolving Credit Facility 1850 Million Capacity [Member]
Line of credit facility [Line Items]
Debt Instrument, Term			5 years
Maximum borrowing capacity			$ 1,850
Line of Credit Facility, Expiration Date			Nov. 30, 2018
Amended Domestic Revolving Credit Facility 2370 Million Capacity [Member]
Line of credit facility [Line Items]
Debt Instrument, Term	5 years
Maximum borrowing capacity	$ 2,370
Line of Credit Facility, Expiration Date		Nov. 30, 2019
Domestic Revolving Credit Facility 165 Million Capacity [Member]
Line of credit facility [Line Items]
Maximum borrowing capacity	$ 165
Line of Credit Facility, Expiration Date	Nov. 30, 2016
Domestic Revolving Credit Facility 20 Million Capacity [Member]
Line of credit facility [Line Items]
Maximum borrowing capacity	$ 20
Line of Credit Facility, Expiration Date	Dec. 31, 2016