Stock-Based Compensation - Restricted Stock Activity and Incentive Stock Awards Activity (Detail) - $ / shares	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Outstanding, beginning of period (in dollars per share)	$ 78.76
Granted (in dollars per share)	73.76
Vested (in dollars per share)	81.42
Adjustment for incentive stock awards expected to vest (in dollars per share)	73.40
Canceled (in dollars per share)	78.80
Outstanding, end of period (in dollars per share)	$ 78.76	$ 75.62
Restricted Stock and Units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, beginning of period (in shares)	399,000
Granted (in shares)	223,990
Vested (in shares)	(143,327)
Adjustment for incentive stock awards expected to vest (in shares)	0
Canceled (in shares)	(28,643)
Outstanding, end of period (in shares)	451,020
Incentive Stock Units
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Outstanding, beginning of period (in shares)	327,333
Granted (in shares)	175,100
Vested (in shares)	(93,312)
Adjustment for incentive stock awards expected to vest (in shares)	(3,934)
Canceled (in shares)	(25,775)
Outstanding, end of period (in shares)	379,412