Stock-Based Compensation (Tables)

12 Months Ended

Dec. 31, 2017

Stock-Based Compensation [Abstract]

Weighted average assumptions used to determine fair value for stock awards

The fair value for stock awards was estimated at the date of grant using the Black-Scholes option valuation model with the following weighted average assumptions for the years ended December 31, 2017, 2016 and 2015:

	2017			2016			2015
Expected life (in years)		6.4			6.5			6.4
Interest rate		2.04	%		1.72	%		1.52	%
Volatility		36.61	%		36.75	%		36.86	%
Expected dividend yield		0.26	%		0.54	%		0.33	%

Summary of stock option activity

The following table summarizes stock option activity for the year ended December 31, 2017:

	Awards			Weighted Average Exercise Price Per Share		Weighted Average Remaining Contractual Life (Years)		Aggregate Intrinsic Value (Millions)
Awards outstanding at December 31, 2016		1,198,725		$	41.66
Granted		187,533			77.99
Exercised		(353,636	)		41.56
Canceled		(35,783	)		50.47
Awards outstanding at December 31, 2017		996,839		$	48.21		6.32	$	22.2
Awards exercisable at December 31, 2017		596,446		$	41.01		4.94	$	16.6

Non-vested stock option activity

Non-vested stock option activity for the year ended December 31, 2017 is as follows:

	Awards			Weighted Average Grant date Fair Value Per Share
Nonvested awards outstanding at December 31, 2016		431,412		$	45.61
Granted		187,533			77.99
Vested		(183,679	)		48.78
Canceled		(34,873	)		50.08
Nonvested awards outstanding at December 31, 2017		400,393		$	58.94

Summary of restricted stock activity

The following table summarizes the restricted stock activity for the Plan:

	Awards			Weighted Average Grant Date Fair Value Per Share
Unvested balance at December 31, 2016		227,213		$	49.57
Granted		69,539			78.00
Vested		(61,274	)		52.51
Canceled		(55,368	)		52.74
Unvested balance at December 31, 2017		180,110		$	58.57