6. Stock-Based Compensation

Total stock-based compensation expense charged as a selling, general and administrative expense in our unaudited consolidated statements of comprehensive income related to our stock compensation plans was $3.0 million for the three months ended March 31, 2014 and March 31, 2013.

The total income tax benefit related to stock-based compensation expense recognized in our unaudited consolidated statements of comprehensive income was $0.9 million and $3.0 million, for the three months ended March 31, 2014 and 2013, respectively.

Presented below is a summary of our stock option activity for the nine months ended March 31, 2014 (shares in thousands):

	Shares		Weighted Average Exercise Price		Weighted Average Remaining Contractual Terms	Aggregate Intrinsic Value

Outstanding at December 31, 2013	4,273		$	17.53
Granted	50		$	20.41
Exercised	(262	)	$	9.67
Forfeited	(143	)	$	23.24
Expired	(8	)	$	26.08
Outstanding at March 31, 2014	3,910		$	17.86	4.73	$	16,661
Expected to vest at March 31, 2014	1,832		$	22.53	6.27	$	1,144
Exercisable at March 31, 2014	2,035		$	13.56	3.27	$	15,485

The fair value of each option grant was estimated using the Black-Scholes and Monte-Carlo option pricing models. Expected volatilities are calculated based on the historical volatility of our common stock. Management monitors stock option exercises and employee termination patterns to estimate forfeiture rates within the valuation model. Separate groups of employees that have similar historical exercise behavior are considered separately for valuation purposes. The expected holding period of options represents the period of time that options granted are expected to be outstanding. The expected terms of options granted are based on our historical experience for similar types of stock option awards. The risk-free interest rate is based on U.S. Treasury Notes.

The weighted average grant-date fair value per share of the stock options granted during the three months ended March 31, 2014 and 2013 was $6.88 and $9.72, respectively. We estimated the fair value of each stock option grant on the date of grant using a Black-Scholes option-pricing model and the weighted-average assumptions set forth in the following table:

	Three months ended March 31
	2014		2013
Expected dividend yield	—		—
Risk-free interest rate	1.65	%	0.67	%
Expected volatility	37.08	%	40.05	%
Expected life	4.81 years		4.47 years

During the three months ended March 31, 2014 and 2013, we issued 0.3 million shares and 0.5 million shares, respectively, of our common stock upon the exercise of outstanding stock options and received proceeds of $2.5 million and $5.4 million, respectively. For the three months ended March 31, 2014 and 2013, we realized a tax benefit of $0.9 million and $3.0 million, respectively from the exercise of stock options.

For the three months ended March 31, 2014, and 2013, approximately $1.9 million, and $1.8 million respectively, of stock-based compensation expense relating to stock options was charged against income. As of March 31, 2014, there was approximately $13.3 million of total unrecognized compensation cost, adjusted for estimated forfeitures, related to stock options outstanding, which is expected to be recognized over a weighted-average period of 1.5 years. As of December 31, 2013, there was approximately $16.0 million of total unrecognized compensation cost, adjusted for estimated forfeitures, related to stock options outstanding, which is expected to be recognized over a weighted-average period of 1.7 years.

The total intrinsic value of options exercised (the difference in the market price of our common stock on the exercise date and the price paid by the optionee to exercise the option) for the three months ended March 31, 2014 and 2013 was approximately $3.0 million and $9.9 million, respectively.

Restricted Stock Units

Our non-employee members of the Board and certain employees have received restricted stock units under our 2006 Stock Plan, as amended. The fair value of restricted stock units is estimated based on the closing sale price of our common stock on the NASDAQ Global Select Market on the date of issuance. The total number of restricted stock units expected to vest is adjusted by estimated forfeiture rates. Shares withheld to pay taxes upon the vesting of restricted stock units are retired.

For the three months ended March 31, 2014, we granted 241,884 restricted stock units with an aggregate fair market value of $4.9 million. At March 31, 2014, 657,486 restricted stock units remained unvested and there was $13.4 million of unamortized compensation cost related to these restricted stock units, which is expected to be recognized over the remaining weighted-average vesting period of 1.9 years. Stock-based compensation expense related to these restricted stock units was $1.0 million for the three months ended March 31, 2014 and 2013.

A summary of the status of our restricted stock units and of changes in restricted stock units outstanding under the 2006 Stock Plan, as amended, for the three months ended March 31, 2014 is as follows (in thousands, except for weighted average grant date fair value per unit):

	Number of Units		Weighted Average Grant Date Fair Value per Unit		Aggregate Intrinsic Value
Outstanding balance at December 31, 2013	636		$	25.50
Granted	242		$	20.26
Vesting of restricted stock units, net of shares withheld for taxes	(50	)	$	25.62
Shares withheld for taxes	(23	)	$	26.22
Forfeitures	(81	)	$	26.22
Outstanding balance at March 31, 2014	724		$	23.66	$	13,800

Restricted Stock Awards

In February 2009, certain of our executive officers received a grant of restricted stock awards under the 2006 Stock Plan, which vested in full in February 2014. The vesting of these restricted stock awards was subject to the executive officers’ continued employment with us and the recipients of these awards were not required to provide us with any consideration other than rendering service.

The stock-based compensation expense for restricted stock awards is determined based on the closing market price of our common stock on the grant date of the awards applied to the total number of awards that are anticipated to fully vest. Shares withheld to pay taxes are retired upon the vesting of the restricted stock awards. We did not issue restricted stock awards during the three months ended March 31, 2014. Stock-based compensation expense related to the previously granted restricted stock awards was $0.1 million and $0.2 million for the three months ended March 31, 2014 and 2013, respectively.

A summary of the status of our restricted stock awards at March 31, 2014 and of changes in restricted stock awards outstanding under the 2006 Stock Plan for the three months ended March 31, 2014 is as follows (in thousands, except for weighted average grant date fair value):

	Shares		Weighted Average Grant Date Fair Value per Share		Aggregate Intrinsic Value
Outstanding balance at December 31, 2013	82		$	10.42
Granted	—		—
Vesting of restricted stock awards	(54	)	$	10.42
Shares withheld for payment of taxes upon vesting of restricted stock awards	(28	)	$	10.42
Forfeitures	—		$	—
Outstanding balance at March 31, 2014	—		$	—	$	—

The total fair value of restricted stock awards vested during the three months ended March 31, 2014 was $0.9 million.