Stock-Based Compensation (Tables)

6 Months Ended

Jun. 30, 2018

Disclosure of Compensation Related Costs, Share-based Payments [Abstract]

Stock-Based Compensation Expense and Related Tax Benefit

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

	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
	(in thousands)
Stock options	$	952	$	598	$	1,783	$	1,257
Restricted stock units	7,365		4,071		14,422		8,368
Unrestricted stock awards	207		255		414		510
Phantom stock units	587		492		1,277		884
Total stock-based compensation	$	9,111	$	5,416	$	17,896	$	11,019
Related tax benefit	$	1,594	$	1,100	$	3,128	$	2,328

Employee Stock Options Activity

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, 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
Outstanding, January 1, 2018	956		$	47.10	6.3	$	21,965
Converted upon acquisition	42		51.86					$	14.86
Granted	101		69.30					$	24.83
Exercised	(87	)	40.08			2,779
Forfeited	(3	)	68.43
Expired	(66	)	95.33
Outstanding, June 30, 2018	943		$	46.92	6.7	$	14,066
Exercisable June 30, 2018	648		$	41.97	5.8	$	12,006
Expected to vest, June 30, 2018	295		$	57.82	8.7	$	2,061

Stock Options, Valuation Assumptions

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,
	2018		2017		2018		2017
Expected volatility	—	%	30.8	%	30.9	%	32.6	%
Risk-free interest rate	—	%	1.8	%	2.8	%	2.0	%
Expected term (years)	N/A		5.5		6.1		5.5

Restricted Stock Units Award Activity

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, 2017	701
Granted	140		$	65.48
Released	(328	)			$	12,533
Forfeited	(19	)
Outstanding, June 30, 2017	494
Outstanding, January 1, 2018	556		$	47.68
Converted upon acquisition	579		69.40
Granted	150		67.61
Released	(445	)	53.01		$	23,609
Forfeited	(79	)	68.56
Outstanding, June 30, 2018	761		62.85
Vested but not released, June 30, 2018	11				$	656
Expected to vest, June 30, 2018	699				$	41,948

Restricted Stock Units, Valuation Assumptions

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,
	2018			2017			2018			2017
Expected volatility	—		%	28.0		%	28.0		%	28.0		%
Risk-free interest rate	—		%	1.4		%	2.2		%	1.1		%
Expected term (years)	N/A			2.5			2.1			1.7
Weighted average fair value	$	—		$	75.58		$	78.56		$	77.65

Schedule of Other Share-based Compensation, Activity

The following table summarizes phantom stock unit activity:

	Number of Phantom Stock Units		Weighted Average Grant Date Fair Value
	(in thousands)
Outstanding, January 1, 2017	62
Granted	32		$	65.55
Released	(20	)
Forfeited	(6	)
Outstanding, June 30, 2017	68
Expected to vest, June 30, 2017	68
Outstanding, January 1, 2018	63		$	51.88
Converted upon acquisition	21		69.40
Granted	32		69.22
Released	(32	)	52.59
Forfeited	(3	)	57.29
Outstanding, June 30, 2018	81		62.76
Expected to vest, June 30, 2018	81