5. Stock Based Compensation

The Company accounts for stock-based compensation pursuant to relevant authoritative guidance, which requires measurement of compensation cost for all stock awards at fair value on the date of grant and recognition of compensation, net of estimated forfeitures, over the requisite service period for awards expected to vest. The Company has Stock Incentive Plans (the Plans) pursuant to which the Company’s board of directors may grant stock options or restricted stock to employees. The Company is authorized to issue grants of restricted stock and stock options to purchase up to 26,963,150 shares for the years ended December 31, 2015, 2014 and 2013, respectively. On May 13, 2013, the Company’s stockholders authorized an increase of 6,500,000 shares of common stock available for grant pursuant to the 2010 Equity Compensation Plan. Giving effect to this increase, there were 4,685,531 additional shares remaining available for grant under the Plans at December 31, 2015.

The table below summarizes the expense recognized related to share-based payments recognized for the years ended December 31 (in thousands):


	2015		2014		2013
Stock options	$	44,260	$	13,267	$	11,677
Restricted stock		45,862		24,382		14,999

Stock-based compensation	$	90,122	$	37,649	$	26,676

The tax benefits recorded on stock based compensation were $35.7 million, $13.0 million and $9.8 million for the years ended December 31, 2015, 2014 and 2013, respectively.

The following table summarizes the Company’s total unrecognized compensation cost related to stock-based compensation as of December 31, 2015 (cost in thousands):


	Unrecognized Compensation Cost		Weighted Average Period of Expense Recognition (in Years)
Stock options	$	44,643		0.89
Restricted stock		7,213		1.24

Total	$	51,856

Stock Options

The following summarizes the changes in the number of shares of common stock under option for the following periods (shares and aggregate intrinsic value in thousands):


	Shares			Weighted Average Exercise Price		Options Exercisable at End of Year		Weighted Average Exercise Price of Exercisable Options		Weighted Average Fair Value of Options Granted During the Year		Aggregate Intrinsic Value
Outstanding at December 31, 2012		6,565		$	22.17		2,666	$	14.71			$	206,636
Granted		307			80.77					$	23.00
Exercised		(1,425	)		21.13								136,807
Forfeited		(116	)		28.68

Outstanding at December 31, 2013		5,331			25.68		2,589		16.57				487,673
Granted		1,544			135.16						42.77
Exercised		(1,429	)		20.75								182,904
Forfeited		(315	)		41.72

Outstanding at December 31, 2014		5,131			58.71		2,370		21.75				461,770
Granted		654			154.56					$	35.32
Exercised		(586	)		33.97								63,863

Forfeited		(196	)		95.16

Outstanding at December 31, 2015		5,003		$	72.72		2,545	$	26.82			$	351,277
Expected to vest at December 31, 2015		5,003		$	72.72

The following table summarizes information about stock options outstanding at December 31, 2015 (shares in thousands):


Exercise Price	Options Outstanding		Weighted Average Remaining Vesting Life in Years		Options Exercisable
$10.00 – 27.83		1,938		0.07		1,713
29.99 – 40.65		995		0.50		712
47.63 – 58.02		5		1.07		—
74.99 – 111.09		231		1.72		55
115.45 – 138.47		347		2.52		63
146.36 – 158.24		1,487		2.44		2

		5,003				2,545

The aggregate intrinsic value of stock options exercisable at December 31, 2015 was $295.5 million. The weighted average remaining contractual term of options exercisable at December 31, 2015 was 5.1 years.

The fair value of stock option awards granted was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions for grants or modifications during the years ended December 31 as follows:


	2015			2014			2013
Risk-free interest rate		1.47	%		1.24	%		0.76	%
Dividend yield		—			—			—
Expected volatility		27.77	%		34.61	%		34.95	%
Expected life (in years)		4.46			3.90			4.00

The weighted-average remaining contractual life for options outstanding was 6.7 and 6.9 years at December 31, 2015 and 2014, respectively.

Restricted Stock

The fair value of restricted stock shares granted based on market conditions was estimated using the Monte Carlo option pricing model with the following assumptions during 2013. There were no restricted stock shares granted with performance based conditions or market conditions in 2015 and 2014.


	2013
Risk-free interest rate		0.42	%
Dividend yield		—
Expected volatility		30.00	%
Expected life (in years)		1.75

The following table summarizes the changes in the number of shares of restricted stock and restricted stock units for the following periods (shares in thousands):


	Shares			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2012		472		$	28.98
Granted		358			92.16
Cancelled		(31	)		35.42
Issued		(165	)		30.93

Outstanding at December 31, 2013		634			67.83
Granted		467			146.12
Cancelled		(76	)		31.48
Issued		(309	)		74.56

Outstanding at December 31, 2014		716			121.38
Granted		126			151.33
Cancelled		(52	)		135.92
Issued		(293	)		85.40

Outstanding at December 31, 2015		497		$	149.40