Stock-Based Compensation (Tables)

9 Months Ended

Sep. 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 September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
	(in thousands)
Stock options	$	911	$	717	$	2,694	$	1,974
Restricted stock units	5,381		4,170		19,803		12,538
Unrestricted stock awards	158		232		572		742
Phantom stock units	762		626		2,039		1,510
Total stock-based compensation	$	7,212	$	5,745	$	25,108	$	16,764
Related tax benefit	$	1,259	$	1,192	$	4,387	$	3,520

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	135		65.94					$	21.98
Exercised	(36	)	38.68			972
Forfeited	(35	)	47.38
Expired	(47	)	67.43
Outstanding, September 30, 2017	976		$	47.60	6.5	$	30,320
Outstanding, January 1, 2018	956		$	47.10	6.3	$	21,965
Converted upon acquisition	42		51.86					$	14.86
Granted	117		68.90					$	24.55
Exercised	(151	)	38.98			4,509
Forfeited	(3	)	68.43
Expired	(66	)	95.31
Outstanding, September 30, 2018	895		$	47.92	6.6	$	15,332
Exercisable September 30, 2018	590		$	42.53	5.6	$	12,867
Expected to vest, September 30, 2018	305		$	58.35	8.5	$	2,466

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 September 30,				Nine Months Ended September 30,
	2018		2017		2018		2017
Expected volatility	28.6	%	29.0	%	30.6	%	32.5	%
Risk-free interest rate	2.9	%	1.8	%	2.8	%	2.0	%
Expected term (years)	6.1		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	141		$	65.54
Released	(340	)			$	13,097
Forfeited	(23	)
Outstanding, September 30, 2017	479
Outstanding, January 1, 2018	556		$	47.68
Converted upon acquisition	579		69.40
Granted	262		65.22
Released	(494	)	54.08		$	26,753
Forfeited	(102	)	68.15
Outstanding, September 30, 2018	801		62.57
Vested but not released, September 30, 2018	9				$	586
Expected to vest, September 30, 2018	741				$	47,554

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 September 30,				Nine Months Ended September 30,
	2018	2017			2018			2017
Expected volatility	N/A	27.9		%	28.0		%	28.0		%
Risk-free interest rate	N/A	1.4		%	2.2		%	1.0		%
Expected term (years)	N/A	2.3			2.1			1.7
Weighted average fair value	N/A	$	80.64		$	78.56		$	77.75

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	(7	)
Outstanding, September 30, 2017	67
Expected to vest, September 30, 2017	67
Outstanding, January 1, 2018	63		$	51.88
Converted upon acquisition	21		69.40
Granted	37		68.06
Released	(34	)	53.55
Forfeited	(3	)	56.47
Outstanding, September 30, 2018	84		62.53
Expected to vest, September 30, 2018	84