Stock-Based Compensation

We maintain the Amended and Restated 2010 Stock Incentive Plan (Stock Incentive Plan), which allows us to grant equity-based compensation awards, including stock options, restricted stock units, phantom stock, and unrestricted stock units. Under the Stock Incentive Plan, we have 7,473,956 shares of common stock reserved and authorized for issuance subject to stock splits, dividends, and other similar events. At March 31, 2017, 1,608,588 shares were available for grant under the Stock Incentive Plan. We issue new shares of common stock upon the exercise of stock options or when vesting conditions on restricted stock units are fully satisfied. These shares are subject to a fungible share provision such that the authorized share reserve is reduced by (i) one share for every one share subject to a stock option or share appreciation right granted under the Plan and (ii) 1.7 shares for every one share of common stock that was subject to an award other than an option or share appreciation right.

We also periodically award phantom stock units, which are settled in cash upon vesting and accounted for as liability-based awards with no impact to the shares available for grant.

In addition, we maintain the Employee Stock Purchase Plan (ESPP), for which approximately 365,000 shares of common stock were available for future issuance at March 31, 2017.

Unrestricted stock and ESPP activity for the quarters ended March 31, 2017 and 2016 was not significant.

Stock-Based Compensation Expense

Total stock-based compensation expense and the related tax benefit were as follows:



	Three Months Ended March 31,
	2017		2016
	(in thousands)
Stock options	$	659	$	554
Restricted stock units	4,297		3,096
Unrestricted stock awards	255		250
Phantom stock units	392		76
Total stock-based compensation	$	5,603	$	3,976

Related tax benefit	$	1,228	$	1,208

Stock Options

A summary of our stock option activity is as follows:



	Shares		Weighted Average Exercise Price per Share		Weighted Average Remaining Contractual Life	Aggregate Intrinsic Value		Weighted Average Grant Date Fair Value
	(in thousands)				(years)	(in thousands)
Outstanding, January 1, 2016	1,180		$	48.31	5.7	$	405
Granted	185		40.04					$	13.15
Exercised	(12	)	35.29			73
Forfeited	(35	)	35.29
Expired	(1	)	48.51
Outstanding, March 31, 2016	1,317		$	47.61	5.5	$	2,859

Outstanding, January 1, 2017	959		$	45.64	6.6	$	19,125
Granted	121		65.55					$	22.01
Exercised	(5	)	35.29			120
Forfeited	—		—
Expired	—		—
Outstanding, March 31, 2017	1,075		$	47.92	6.7	$	17,236

Exercisable March 31, 2017	707		$	48.34	5.5	$	11,657

Expected to vest, March 31, 2017	368		$	47.13	9.0	$	5,579

At March 31, 2017, total unrecognized stock-based compensation expense related to nonvested stock options was $5.1 million, which is expected to be recognized over a weighted average period of approximately 2.0 years.

The weighted-average assumptions used to estimate the fair value of stock options granted and the resulting weighted average fair value are as follows:



	Three Months Ended March 31,
	2017			2016
Expected volatility	32.7		%	33.5		%
Risk-free interest rate	2.0		%	1.3		%
Expected term (years)	5.5			5.5

Weighted average fair value	$	22.01		$	13.15

Restricted Stock Units

The following table summarizes restricted stock unit activity:



	Number of Restricted Stock Units		Weighted Average Grant Date Fair Value		Aggregate Intrinsic Value
	(in thousands)				(in thousands)
Outstanding, January 1, 2016	756
Granted	172		$	40.02
Released	(262	)			$	10,098
Forfeited	(30	)
Outstanding, March 31, 2016	636

Outstanding, January 1, 2017	701		$	38.04
Granted	131		63.12
Released	(317	)	38.13		$	12,066
Forfeited	(3	)	35.68
Outstanding, March 31, 2017	512		44.45

Vested but not released, March 31, 2017	6				$	374

Expected to vest, March 31, 2017	410				$	24,894

At March 31, 2017, total unrecognized compensation expense on restricted stock units was $35.5 million, which is expected to be recognized over a weighted average period of approximately 2.1 years.

The weighted-average assumptions used to estimate the fair value of performance-based restricted stock units granted and the resulting weighted average fair value are as follows:



	Three Months Ended March 31,
	2017			2016
Expected volatility	28.0		%	30.0		%
Risk-free interest rate	1.0		%	0.7		%
Expected term (years)	1.7			1.8

Weighted average fair value	$	77.78		$	44.77

Phantom Stock Units

The following table summarizes phantom stock unit activity:



	Number of Phantom Stock Units		Weighted Average Grant Date Fair Value
	(in thousands)
Outstanding, January 1, 2016	—
Granted	61		$	41.72
Forfeited	—
Outstanding, March 31, 2016	61

Expected to vest, March 31, 2016	53

Outstanding, January 1, 2017	62		$	40.11
Granted	32		65.55
Released	(19	)	40.05
Forfeited	(2	)	40.05
Outstanding, March 31, 2017	73		51.40

Expected to vest, March 31, 2017	73

At March 31, 2017, total unrecognized compensation expense on phantom stock units was $4.2 million, which is expected to be recognized over a weighted average period of approximately 2.4 years. We have recognized a phantom stock liability of $0.2 million within wages and benefits payable in the Consolidated Balance Sheets as of March 31, 2017.