Stock-Based Compensation - Additional Information (Detail) $ / shares in Units, shares in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Employee stock options, granted | shares	0.2
Employee stock options, weighted average grant date fair value | $ / shares	$ 45.87
Employee Stock Options [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options, award vesting period (in years)	1 year 4 months 26 days
Unrecognized compensation expense | $	$ 9.7
Weighted average period to recognize expense	4 years
Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options, Shares granted | shares	0.7
Other than options, weighted average grant date fair value | $ / shares	$ 167.48
Other than options, award vesting period (in years)	1 year 8 months 12 days
Unrecognized compensation expense | $	$ 191.4
Weighted average period to recognize expense	4 years
Performance Based Restricted Stock [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Other than options, Shares granted | shares	0.1
Other than options, weighted average grant date fair value | $ / shares	$ 162.42
Other than options, award vesting period (in years)	1 year
Unrecognized compensation expense | $	$ 37.5
Weighted average period to recognize expense	3 years