Stock-Based Compensation

We maintain the Second Amended and Restated 2010 Stock Incentive Plan (Stock Incentive Plan), which allows us to grant stock-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 December 31, 2017, 4,656,327 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 ESPP, for which approximately 340,000 shares of common stock were available for future issuance at December 31, 2017.

Unrestricted stock and ESPP activity for the years ended December 31, 2017, 2016, and 2015 was not significant.

Stock-Based Compensation Expense

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



	2017		2016		2015
	(in thousands)
Stock options	$	2,695	$	2,357	$	2,648
Restricted stock units	17,738		14,723		10,735
Unrestricted stock awards	974		955		706
Phantom stock units	1,747		1,077		—
Total stock-based compensation	$	23,154	$	19,112	$	14,089

Related tax benefit	$	5,034	$	4,927	$	4,228

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, 2015	1,123		$	51.90	4.4	$	1,676
Granted	291		35.25					$	12.09
Exercised	(24	)	36.05			26
Forfeited	(17	)	37.47
Expired	(193	)	52.17
Outstanding, December 31, 2015	1,180		$	48.31	5.7	$	405

Granted	191		$	40.40				$	13.27
Exercised	(58	)	37.00			$	742
Forfeited	(36	)	35.29
Expired	(318	)	55.13
Outstanding, December 31, 2016	959		$	45.64	6.6	$	19,125

Granted	135		$	65.94				$	21.99
Exercised	(41	)	39.92			$	1,071
Forfeited	(35	)	47.38
Expired	(62	)	70.12
Outstanding, December 31, 2017	956		$	47.10	6.3	$	21,965

Exercisable, December 31, 2017	640		$	46.08	5.3	$	15,934

Expected to vest, December 31, 2017	316		$	49.17	8.4	$	6,031

At December 31, 2017, total unrecognized stock-based compensation expense related to nonvested stock options was $2.9 million, which is expected to be recognized over a weighted average period of approximately 1.5 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:



	Year Ended December 31,
	2017		2016		2015
Expected volatility	32.5	%	33.5	%	34.3	%
Risk-free interest rate	2.0	%	1.3	%	1.7	%
Expected term (years)	5.5		5.5		5.5

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, 2015	682
Granted	434		$	35.09
Released	(296	)			$	12,204
Forfeited	(64	)
Outstanding, December 31, 2015	756

Granted	306		$	41.58
Released	(312	)			$	11,944
Forfeited	(49	)
Outstanding, December 31, 2016	701		$	38.04

Granted	273		$	50.95
Released	(372	)	36.93		$	14,219
Forfeited	(46	)	48.56
Outstanding, December 31, 2017	556		$	47.68

Vested but not released, December 31, 2017	142				$	9,650

Expected to vest, December 31, 2017	350				$	23,877

At December 31, 2017, total unrecognized compensation expense on restricted stock units was $21.7 million, which is expected to be recognized over a weighted average period of approximately 1.6 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:



	Year Ended December 31,
	2017			2016			2015
Expected volatility	28.0		%	30.0		%	30.1		%
Risk-free interest rate	1.0		%	0.7		%	0.7		%
Expected term (years)	1.7			1.8			2.1

Weighted average grant date fair value	$	77.75		$	44.92		$	33.48

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, December 31, 2016	62

Expected to vest, December 31, 2016	57

Outstanding, January 1, 2017	62		$	40.11
Granted	32		65.55
Released	(20	)	47.02
Forfeited	(11	)	40.11
Outstanding, December 31, 2017	63		$	47.28

Expected to vest, December 31, 2017	63

At December 31, 2017, total unrecognized compensation expense on phantom stock units was $2.8 million, which is expected to be recognized over a weighted average period of approximately 1.7 years. As of December 31, 2017 and 2016, we have recognized a phantom stock liability of $1.7 million and $1.0 million, respectively, within wages and benefits payable in the Consolidated Balance Sheets.