Share-Based Compensation (Tables)

9 Months Ended

12 Months Ended

Sep. 30, 2018

Dec. 31, 2017

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

Summary of Performance Based Stock Options

Performance-based option activity under the 2007 SIP and the 2013 SIP for the period was as follows:


	Shares			Weighted Average Exercise Price (per share)			Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Performance-based options outstanding at December 31, 2017		1,038,147		$	13.46			3.5	$	—
Granted		—			—			—		—
Exercised		(633,606	)		(13.46	)		—		—
Forfeited or expired		(8,358	)		(13.46	)		—		—

Performance-based options outstanding at September 30, 2018		396,183		$	13.50			3.4	$	11.3
Performance-based options exercisable at September 30, 2018		396,183		$	13.50			3.4	$	11.3

Performance-based option activity for the period from December 31, 2014 to December 31, 2017, represents stock options granted under the 2013 HCM SIP.


	Shares			Weighted Average Exercise Price (per share)			Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Options outstanding at December 31, 2014		1,343,100		$	13.46			6.3	$	—
Granted		—			—
Exercised		—			—
Forfeited or expired		(62,232	)		(13.46	)

Options outstanding at December 31, 2015		1,280,868		$	13.46			5.2	$	—
Granted		—			—
Exercised		—			—
Forfeited or expired		(52,942	)		(13.46	)

Options outstanding at December 31, 2016		1,227,926		$	13.46			4.2	$	—
Granted		—			—
Exercised		(167,202	)(a)		(13.46	)
Forfeited or expired		(25,077	)		(13.46	)

Options outstanding at December 31, 2017		1,035,647		$	13.46			3.5	$	—

(a)	During the year ended December 31, 2017, certain performance-based options were modified and exercised.

Summary of Term-Based Option Activity

Term-based option activity, including stock options under the 2007 SIP, the 2013 SIP and the 2018 EIP, for the period was as follows:


	Shares			Weighted Average Exercise Price (per share)			Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Term-based options outstanding at December 31, 2017		10,994,181		$	16.52			6.9	$	48.8
Granted		5,140,787			22.84			—		—
Exercised		(1,127,360	)		(13.73	)		—		—
Forfeited or expired		(166,747	)		(17.09	)		—		—

Term-based options outstanding at September 30, 2018		14,840,861		$	18.66			7.6	$	346.8
Term-based options exercisable at September 30, 2018		6,868,950		$	16.70			5.8	$	174.0

Term-based option activity for the period from December 31, 2014 to December 31, 2017, represents stock options granted under the 2013 HCM SIP. As of December 31, 2017, there were 6,819,649 vested options.


	Shares			Weighted Average Exercise Price (per share)			Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value (in millions)
Options outstanding at December 31, 2014		7,588,078		$	15.98			8.2	$	13.3
Granted		187,140			17.56
Exercised		—			—
Forfeited or expired		(168,060	)		(16.40	)

Options outstanding at December 31, 2015		7,607,158		$	16.02			7.3	$	14.1
Granted		2,339,238			16.80
Exercised		—			—
Forfeited or expired		(176,626	)		(16.54	)

Options outstanding at December 31, 2016		9,769,770		$	16.20			7.1	$	9.9
Granted		2,285,981			17.46
Exercised		(595,464	)		(15.14	)
Forfeited or expired		(468,606	)		(16.10	)

Options outstanding at December 31, 2017		10,991,681		$	16.52			6.9	$	48.8
Options exercisable at December 31, 2017		6,819,649		$	16.14			5.5	$	32.9

Summary of Restricted Stock Units

Restricted stock units (“RSUs”) activity, including RSUs under the 2013 SIP and the 2018 EIP, for the period was as follows:


	Shares
RSUs outstanding at December 31, 2017		605,990
Granted		159,323
Shares issued upon vesting of RSUs		(105,990	)
Forfeited or canceled		—

RSUs outstanding at September 30, 2018		659,323
RSUs releasable at September 30, 2018		125,000

Restricted stock units (“RSUs”) activity for the period from December 31, 2014 to December 31, 2017, represents restricted stock units granted under the 2013 HCM SIP.


	Shares
RSUs outstanding at December 31, 2014		—
Granted		228,572
Shares issued upon vesting of RSUs		—
Forfeited or canceled		—

RSUs outstanding at December 31, 2015		228,572
Granted		29,800
Shares issued upon vesting of RSUs		(76,192	)
Forfeited or canceled		—

RSUs outstanding at December 31, 2016		182,180
Granted		500,000
Shares issued upon vesting of RSUs		(76,190	)
Forfeited or canceled		—

RSUs outstanding at December 31, 2017		605,990

Schedule of Other Term-Based Option Activity

Other information pertaining to term-based options is as follows:


	Year Ended December 31,
	2017		2016		2015
Weighted average grant date fair value per share	$	5.88	$	5.74	$	5.68

Schedule of Other Term-Based Option Activity

The fair value of the term-based stock options was estimated at the date of grant using the Black-Scholes option pricing model with the following weighted-average assumptions:


	Year Ended December 31,
	2017			2016			2015
Expected volatility		30.0	%		30.0	%		33.0	%
Expected dividend rate		—			—			—
Risk-free interest rate		2.3	%		1.9	%		2.1	%

Schedule of Other Class D Stock Options

Other information pertaining to restricted stock units is as follows:


	Year Ended December 31,
	2017		2016		2015
Weighted average grant date fair value per share	$	17.26	$	17.40	$	17.50