Stock-Based Compensation

We maintain the Second 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 10,473,956 shares of common stock reserved and authorized for issuance subject to stock splits, dividends, and other similar events. At June 30, 2017, 4,647,815 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 355,000 shares of common stock were available for future issuance at June 30, 2017.

Unrestricted stock and ESPP activity for the three and six months ended June 30, 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 June 30,				Six Months Ended June 30,
	2017		2016		2017		2016
	(in thousands)
Stock options	$	598	$	585	$	1,257	$	1,139
Restricted stock units	4,071		3,143		8,368		6,239
Unrestricted stock awards	255		250		510		500
Phantom stock units	492		211		884		287
Total stock-based compensation	$	5,416	$	4,189	$	11,019	$	8,165

Related tax benefit	$	1,100	$	1,296	$	2,328	$	2,504

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	(34	)	35.29			195
Forfeited	(36	)	35.29
Expired	(147	)	62.50
Outstanding, June 30, 2016	1,148		$	45.95	6.0	$	3,700

Outstanding, January 1, 2017	959		$	45.64	6.6	$	19,125
Granted	132		65.80					$	21.99
Exercised	(34	)	37.58			933
Forfeited	(35	)	47.38
Expired	(47	)	67.43
Outstanding, June 30, 2017	975		$	47.54	6.7	$	21,680

Exercisable June 30, 2017	632		$	47.38	5.5	$	14,846

Expected to vest, June 30, 2017	343		$	47.85	8.8	$	6,834

At June 30, 2017, total unrecognized stock-based compensation expense related to nonvested stock options was $4.2 million, which is expected to be recognized over a weighted average period of approximately 1.8 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 June 30,						Six Months Ended June 30,
	2017			2016			2017			2016
Expected volatility	30.8		%	—		%	32.6		%	33.5		%
Risk-free interest rate	1.8		%	—		%	2.0		%	1.3		%
Expected term (years)	5.5			—			5.5			5.5

Weighted average fair value	$	21.71		$	—		$	21.99		$	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	(270	)			$	10,429
Forfeited	(42	)
Outstanding, June 30, 2016	616

Outstanding, January 1, 2017	701		$	38.04
Granted	140		65.48
Released	(328	)	38.26		$	12,533
Forfeited	(19	)	43.60
Outstanding, June 30, 2017	494		45.58

Vested but not released, June 30, 2017	7				$	447

Expected to vest, June 30, 2017	403				$	27,332

At June 30, 2017, total unrecognized compensation expense on restricted stock units was $30.2 million, which is expected to be recognized over a weighted average period of approximately 1.9 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 June 30,						Six Months Ended June 30,
	2017			2016			2017			2016
Expected volatility	28.0		%	—		%	28.0		%	30.0		%
Risk-free interest rate	1.4		%	—		%	1.1		%	0.7		%
Expected term (years)	2.5			—			1.7			1.8

Weighted average fair value	$	75.58		$	—		$	77.65		$	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	63		$	40.11
Forfeited	(1	)
Outstanding, June 30, 2016	62

Expected to vest, June 30, 2016	55

Outstanding, January 1, 2017	62		$	40.11
Granted	32		65.55
Released	(20	)	40.11
Forfeited	(6	)	40.05
Outstanding, June 30, 2017	68		52.18

Expected to vest, June 30, 2017	68

At June 30, 2017, total unrecognized compensation expense on phantom stock units was $3.9 million, which is expected to be recognized over a weighted average period of approximately 2.0 years. As of June 30, 2017 and December 31, 2016, we have recognized a phantom stock liability of $0.7 million and $1.0 million, respectively, within wages and benefits payable in the Consolidated Balance Sheets.