Debt and Lines of Credit - Additional Information (Details) - USD ($)	1 Months Ended			3 Months Ended		9 Months Ended
	Aug. 31, 2015	May. 31, 2015	Apr. 30, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Debt Instrument [Line Items]
Commercial paper				$ 0		$ 0
Long-term debt, gross				4,125,000,000		4,125,000,000		$ 4,625,000,000
Proceeds from issuance of long-term debt		$ 498,000,000				498,000,000	$ 498,000,000
Interest and debt expense				22,000,000	$ 23,000,000	68,000,000	$ 72,000,000
Notes payable
Debt Instrument [Line Items]
Retirement of long-term debt	$ 750,000,000		$ 250,000,000
Long-term debt, gross		500,000,000
Payments of debt issuance costs		$ 3,000,000
Consortium of investment-grade banks | Revolving credit facility
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity until March 2020				$ 2,000,000,000		$ 2,000,000,000