Stock-based compensation expense recognized in the Consolidated Statements of Earnings totaled $7.3 million and $6.2 million for the three months ended June 30, 2018 and 2017, respectively. For the six months ended June 30, 2018 and 2017, stock-based compensation expense was $14.3 million and $12.1 million, respectively.

Stock Options

The expense related to stock options granted in the six months ended June 30, 2018 and 2017 was estimated on the date of grant using a Black-Scholes option-pricing model based on the assumptions shown in the table below:



	Six Months Ended June 30,
	2018			2017
Risk-free interest rate	2.59%			1.73%	to	1.93%
Dividend yield	—%			—%
Expected life (years)	4.5			4.5
Volatility	41.2%			38.1%	to	38.8%
Fair value at date of grant	$4.83	to	$5.39	$6.07	to	$6.73

The following table summarizes the Company's stock-settled stock appreciation right ("SSAR") and stock option activity for the six months ended June 30, 2018 (in millions, except share and per share amounts):



	SSARs						Stock Options
	Number of Shares		Weighted-Average Exercise Price		Aggregate Intrinsic Value	Weighted-Average Remaining Contractual Term (Years)	Number of Shares		Weighted-Average Exercise Price		Aggregate Intrinsic Value	Weighted-Average Remaining Contractual Term (Years)
Outstanding at December 31, 2017	850,516		$	21.54			4,901,739		$	18.36
Granted	—		—				923,283		14.28
Exercised	(30,624	)	12.26				(26,595	)	11.02
Forfeited	—		—				(47,918	)	15.91
Expired	(37,897	)	22.79				(140,839	)	21.27
Outstanding at June 30, 2018	781,995		$	21.85	$0.1	3.6	5,609,670		$	17.67	$7.5	4.6

Exercisable at June 30, 2018	781,995		$	21.85	$0.1	3.6	3,555,319		$	19.37	$4.3	3.9

There was no unrecognized compensation expense related to SSARs at June 30, 2018. At June 30, 2018, unrecognized compensation expense related to stock options not yet exercisable of $8.6 million is expected to be recognized over a weighted-average period of 1.4 years.

RSUs

The following table summarizes the Company's restricted stock unit ("RSU") balances for the six months ended June 30, 2018:



	Share units		Weighted-average grant date fair value
Unvested at December 31, 2017	2,202,576		$	16.54
Granted	1,569,435		14.14
Vested	(896,114	)	17.10
Forfeited	(193,995	)	15.37
Unvested at June 30, 2018	2,681,902		$	15.05

At June 30, 2018, $33.0 million of unrecognized compensation expense related to RSUs is expected to be recognized over a weighted-average period of 1.5 years.

PSUs

In February 2018 and 2017, the Company granted performance stock units ("PSUs") to senior management. In each case, the awards will cliff vest three years following the grant date and the number of PSUs that may be earned and vest is based on the Company's revenues and stock price performance over a three year performance period. PSUs will be settled in shares of the Company's common stock. Depending on the Company's overall performance relative to revenues and stock price, the size of the PSU awards are subject to adjustment, up or down, resulting in awards at the end of the performance period that can range from 0% to 225% of the initial grant value. The Company will ratably recognize the expense over the applicable service period for each grant of PSUs and adjust the expense as appropriate. The fair value of the PSUs is determined by using a Monte Carlo simulation.

The following table summarizes the Company's PSU balances for the six months ended June 30, 2018:



	Share units	Weighted-average grant date fair value
Unvested at December 31, 2017	176,000	$	15.32
Granted	381,967	13.78
Vested	—	—
Forfeited	—	—
Unvested at June 30, 2018	557,967	$	14.27

At June 30, 2018, $8.4 million of unrecognized compensation expense related to PSUs is expected to be recognized over a weighted-average period of 2.4 years.