The Company's 2015 Stock Incentive Plan (the "2015 Plan"), which provides for share-based awards to Company employees, non-employee directors and outside consultants or advisors, was approved by stockholders on May 27, 2015. The number of shares of common stock reserved for issuance under the 2015 Plan is 1,000,000, of which the maximum number of shares for which incentive stock options may be granted is 900,000. The 2015 Plan replaced the Company’s 2009 Stock Incentive Plan (the “2009 Plan”), and no new share-based awards may be granted under the 2009 Plan. The number of shares available for grant at December 31, 2019 is 427,408.

Share-based compensation expense included in selling, general and administrative expenses during the years ended December 31, 2019, 2018 and 2017, was $7.2 million, $5.5 million and $4.2 million, respectively. Related income tax benefits for the years ended December 31, 2019, 2018 and 2017, were $1.3 million, $1.5 million and $1.5 million, respectively.

Outstanding stock options generally vest over either four or eight years and expire ten years after the date of grant.

A summary of the status of the Company’s stock options at December 31, 2019, together with changes during the periods then ended, is presented below:

						Weighted
				Weighted		Average		Aggregate
				Average		Remaining		Intrinsic
	Number			Exercise		Contractual		Value
	of Options			Price		Term (Years)		(In Thousands)
Outstanding at December 31, 2018		372,260		$	39.08		—		—
Options exercised		(54,420	)		13.87		—		—
Outstanding at December 31, 2019		317,840			43.40		4.49		14,958
Exercisable at December 31, 2019		195,340			19.60		2.16	$	13,841

The fair value of stock option awards as determined under the Black-Scholes option-pricing model was estimated with the following weighted-average assumptions:

	2018
Expected volatility		42.2	%
Risk free interest rate		2.7	%
Expected dividend yield		1.2	%
Expected term		8.3 years
Weighted average fair value per share	$	36.41

No stock options were granted during the years ended December 31, 2019 and 2017. The weighted average fair value per share of stock options granted for the year ended December 31, 2018 was $36.41. No stock options were granted with an exercise price below market price during 2018.

The intrinsic value of stock options exercised for the years ended December 31, 2019, 2018 and 2017 was $3.8 million, $2.5 million, and $479,000, respectively. The fair value of stock options vested for the years ended December 31, 2019, 2018 and 2017 was $312,000, $312,000, and $328,000, respectively. As of December 31, 2019, unrecognized compensation expense related to stock options was $3.9 million with a weighted average remaining amortization period of 6.2 years.

Restricted stock units generally vest in four equal annual installments beginning one year following the date of grant.

				Weighted Average
				Grant Date
	Units			Fair Value
Nonvested at December 31, 2018		203,854		$	60.93
Granted		72,803			79.55
Vested		(98,829	)		56.49
Cancelled/Forfeited		(4,355	)		70.12
Nonvested at December 31, 2019		173,473		$	71.49

The total fair value of restricted stock units vested during the years ended December 31, 2019, 2018 and 2017 was $5.8 million, $4.0 million and $3.3 million, respectively. As of December 31, 2019, unrecognized compensation expense related to restricted stock units was $9.7 million with a weighted average remaining amortization period of 2.6 years.

Performance share units (“PSUs”) are granted to key employees of the Company and are conditioned on attaining specified financial performance metrics. Awards are subject to upward or downward adjustments depending on whether the actual financial metrics are above or below the target level, with a maximum payout up to 200% of a target number of shares.

PSUs fully vest on the date that the Compensation Committee determines the level of attainment of specified performance goals, generally over a three-year period.

				Weighted Average
				Grant Date
	Units			Fair Value
Nonvested at December 31, 2018		33,325		$	66.34
Granted		11,488			72.02
Vested		(3,969	)		74.60
Cancelled/Forfeited		(5,018	)		64.76
Nonvested at December 31, 2019		35,826		$	67.47

The total fair value of PSUs vested during the year ended December 31, 2019 was $296,000. No PSUs vested in 2018 and 2017.

The Company offers employees the right to purchase shares at a discount from the market price under the Company’s 2019 Employee Stock Purchase Plan. Subject to the annual statutory limit, employees are eligible to participate through payroll deductions of up to 15% of their compensation. At the end of each six-month offering period, shares are purchased by the participants at 85% of the fair market value at the end of the offering period. As of December 31, 2019, approximately 300,000 shares were reserved for future issuance under the 2019 Employee Stock Purchase Plan.