Share-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

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

Schedule of Stock Option Activity

The following table presents a summary of option activity and data under the Company’s stock incentive plans as of December 31, 2017:

	Number of Shares		Weighted-Average Exercise Price Per Share		Weighted- Average Remaining Contractual Term (Years)	Aggregate Intrinsic Value
Balance at January 1, 2017	8,023,696		$	28.27
Granted	1,693,309		$	49.89
Exercised	(1,854,644	)	$	24.57
Forfeited and expired	(818,871	)	$	37.24
Outstanding, December 31, 2017	7,043,490		$	33.40	5.85	$	11,705,399
Vested and expected to vest, December 31, 2017	6,842,097		$	33.13	5.81	$	11,699,950
Exercisable, December 31, 2017	4,190,582		$	28.97	5.05	$	11,487,732
Available for future grant at December 31, 2017	3,202,401

Schedule of Valuation Assumptions

The Company estimated the fair value of each option on the date of grant using the Black-Scholes closed-form option-pricing model applying the weighted average assumptions in the following table.

	Years Ended December 31,
	2017		2016		2015
Expected dividend yield	—	%	—	%	—	%
Expected stock price volatility	39.14	%	37.90	%	41.49	%
Risk-free interest rate	1.867	%	1.249	%	1.436	%
Expected option term (years)	5.00		4.93		5.01

The fair value of each option element of the 2010 ESPP is estimated on the date of grant using the Black-Scholes closed-form option-pricing model applying the weighted average assumptions in the following table. Expected volatilities are based on historical volatility of the Company’s common stock. Expected term represents the six-month offering period for the 2010 ESPP. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant.

	Years Ended December 31,
	2017		2016		2015
Expected dividend yield	—	%	—	%	—	%
Expected stock price volatility	43.03	%	48.80	%	44.91	%
Risk-free interest rate	0.89	%	0.34	%	0.15	%
Expected option term (years)	0.5		0.5		0.5

Schedule of Restricted Stock and Restricted Stock Units Activity

The following table presents a summary of the Company’s outstanding shares of restricted stock awards granted as of December 31, 2017:

	Number of Shares		Weighted Average Grant-Date Fair Value
Balance at January 1, 2017	375,301		$	31.88
Awarded	196,462		50.51
Vested	(172,076	)	32.80
Forfeited	(30,359	)	44.03
Outstanding, December 31, 2017	369,328		$	40.37