Fair Value Measurements (Tables)

12 Months Ended

Dec. 31, 2017

Fair Value Disclosures [Abstract]

Schedule of assets and liabilities measured at fair value on a recurring basis

Our assets and liabilities measured at fair value on a recurring basis were as follows:

	Quoted Prices in Active Markets for Identical Assets (Level 1)				Significant Other Observable Inputs (Level 2)						Significant Unobservable Inputs (Level 3)				Total
(millions)	12/31/17		12/31/16		12/31/17			12/31/16			12/31/17		12/31/16		12/31/17			12/31/16
Cash equivalents	$	124	$	38	$	24		$	34		$	—	$	—	$	148		$	72
Equity mutual funds	6		5		—			—			—		—		6			5
Marketable securities:
Corporate debt securities	—		—		68			69			—		—		68			69
U.S. government and agency debt securities	—		—		6			14			—		—		6			14
Asset-backed debt securities	—		—		11			2			—		—		11			2
Certificates of deposit	—		—		13			6			—		—		13			6
Municipal debt securities	—		—		1			—			—		—		1			—
Derivative assets	—		—		2			13			—		—		2			13
Derivative liabilities	—		—		(17		)	(10		)	—		—		(17		)	(10		)