Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Weighted average grant date fair values of MSUs

We granted MSUs with the following weighted average grant date fair values:

	2017		2016		2015
Weighted average grant date fair values	$	35.79	$	19.59	$	30.06

Schedule of MSUs valuation assumptions

We estimated the fair value of each MSU granted on the date of grant using a Monte Carlo simulation that used the assumptions noted in the following table. Volatility was based on stock price history immediately prior to grant for a period commensurate with the expected term. The risk-free rate was based on zero-coupon U.S. government issues at the time of grant. The expected term represents the period from the valuation date to the end of the performance period.

Assumptions:	2017		2016		2015
Expected volatility	32.10	%	34.02	%	42.70	%
Risk-free rate	1.39	%	0.86	%	1.09	%
Expected term (in years)	2.96		2.95		2.95
Expected dividends	—		—		—

Schedule of nonvested MSUs outstanding and activity

Nonvested MSUs outstanding as of December 31, 2017 and MSU activity during 2017 were as follows:

	Number of MSUs (000)		Weighted Average Grant Date Fair Value
Nonvested at January 1, 2017	1,157		$	23.39
Granted	371		35.79
Vested	(417	)	30.06
Forfeited	(14	)	26.20
Nonvested at December 31, 2017	1,097		25.01

Weighted average grant date fair values of performance shares

We granted performance shares with the following weighted average grant date fair values:

	2017		2016		2015
Weighted average grant date fair values	$	39.42	$	21.10	$	30.63

Schedule of performance based units valuation assumptions

We estimated the fair value of each performance share granted on the date of grant using a Monte Carlo simulation that uses the assumptions noted in the following table. Volatility was based on stock price history immediately prior to grant for a period commensurate with the expected term. The risk-free rate was based on zero-coupon U.S. government issues at the time of grant. The expected term represents the period from the grant date to the end of the three-year performance period.

Assumptions:	2017		2016		2015
Expected volatility	32.10	%	34.02	%	42.70	%
Risk-free rate	1.39	%	0.86	%	1.09	%
Expected term (in years)	2.96		2.95		2.95
Expected dividends	—		—		—

Summary of nonvested performance share outstanding and performance share activity

Nonvested performance shares outstanding as of December 31, 2017 and performance share activity during 2017 were as follows:

	Number of Performance Shares (000)		Weighted Average Grant Date Fair Value
Nonvested at January 1, 2017	246		$	24.98
Granted	113		39.42
Vested	(99	)	30.63
Forfeited	(4	)	26.75
Nonvested at December 31, 2017	256		29.14

Weighted average grant date fair values of RSUs

We granted RSUs with the following weighted average grant date fair values:

	2017		2016		2015
Weighted average grant date fair values	$	31.57	$	23.94	$	28.56

Summary of nonvested RSUs outstanding and activity

RSUs outstanding as of December 31, 2017 and RSU activity during the year then ended were as follows:

	Number of RSUs (000)		Weighted Average Grant Date Fair Value
Nonvested at January 1, 2017	228		$	27.31
Granted	133		31.57
Vested	(54	)	31.38
Forfeited	(3	)	29.83
Nonvested at December 31, 2017	304		14.67

Summary of stock options outstanding and stock option activity

A summary of stock options outstanding as of December 31, 2017 and of stock option activity during the year then ended is presented below:

	Number of Options (000)		Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (years)	Aggregate Intrinsic Value (millions)
Outstanding at January 1, 2017	2,174		$	22.55	2.78	$	22
Exercised	(590	)	23.88
Canceled	(323	)	46.80
Outstanding at December 31, 2017	1,261		$	15.72	2.48	$	28
Exercisable at December 31, 2017	1,261		$	15.72	2.48	$	28
Vested at December 31, 2017	1,261		$	15.72	2.48	$	28

(millions)	2017		2016		2015
Intrinsic value of stock options exercised	$	7	$	4	$	6
Cash received from stock options exercised	$	14	$	4	$	6
Fair value of stock options vested	$	—	$	1	$	1