Share-based Compensation - Schedule of Stock Options Outstanding and Exercisable by Exercise Price Range (Detail)	12 Months Ended
	Dec. 31, 2017 $ / shares shares
$21.78 – $27.97 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	$ 21.78
Exercise price, high end of range (in dollars per share)	$ 27.97
Number outstanding (in shares) | shares	111,874
Weighted Average Remaining Contractual Life of options outstanding (in years)	5 years 29 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 24.12
Number Exercisable (in shares) | shares	111,874
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 24.12
$27.98 – $32.28 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	27.98
Exercise price, high end of range (in dollars per share)	$ 32.28
Number outstanding (in shares) | shares	110,400
Weighted Average Remaining Contractual Life of options outstanding (in years)	5 years 4 months 9 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 31.12
Number Exercisable (in shares) | shares	91,326
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 30.90
$33.12 – $33.12 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	33.12
Exercise price, high end of range (in dollars per share)	$ 33.12
Number outstanding (in shares) | shares	140,475
Weighted Average Remaining Contractual Life of options outstanding (in years)	7 years 6 months
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 33.12
Number Exercisable (in shares) | shares	70,242
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 33.12
$33.24 – $36.25 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	33.24
Exercise price, high end of range (in dollars per share)	$ 36.25
Number outstanding (in shares) | shares	121,875
Weighted Average Remaining Contractual Life of options outstanding (in years)	6 years 8 months 26 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 35.51
Number Exercisable (in shares) | shares	83,213
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 35.71
$36.46 – $38.40 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	36.46
Exercise price, high end of range (in dollars per share)	$ 38.40
Number outstanding (in shares) | shares	39,750
Weighted Average Remaining Contractual Life of options outstanding (in years)	8 years 6 months 18 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 37.92
Number Exercisable (in shares) | shares	12,376
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 37.64
$38.82 – $38.82 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	38.82
Exercise price, high end of range (in dollars per share)	$ 38.82
Number outstanding (in shares) | shares	150,000
Weighted Average Remaining Contractual Life of options outstanding (in years)	5 years 2 months 12 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 38.82
Number Exercisable (in shares) | shares	150,000
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 38.82
$39.66 – $41.37 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	39.66
Exercise price, high end of range (in dollars per share)	$ 41.37
Number outstanding (in shares) | shares	65,500
Weighted Average Remaining Contractual Life of options outstanding (in years)	6 years 1 month 13 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 40.23
Number Exercisable (in shares) | shares	43,000
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 40.25
$42.89 – $42.89 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	42.89
Exercise price, high end of range (in dollars per share)	$ 42.89
Number outstanding (in shares) | shares	22,000
Weighted Average Remaining Contractual Life of options outstanding (in years)	8 years 8 months 26 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 42.89
Number Exercisable (in shares) | shares	5,500
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 42.89
$44.39 – $44.39 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	44.39
Exercise price, high end of range (in dollars per share)	$ 44.39
Number outstanding (in shares) | shares	163,138
Weighted Average Remaining Contractual Life of options outstanding (in years)	8 years 6 months
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 44.39
Number Exercisable (in shares) | shares	40,789
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 44.39
$46.10 – $46.10 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	46.10
Exercise price, high end of range (in dollars per share)	$ 46.10
Number outstanding (in shares) | shares	161,810
Weighted Average Remaining Contractual Life of options outstanding (in years)	9 years 6 months
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 46.10
$21.78 – $46.10 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Exercise price, low end of range (in dollars per share)	21.78
Exercise price, high end of range (in dollars per share)	$ 46.10
Number outstanding (in shares) | shares	1,086,822
Weighted Average Remaining Contractual Life of options outstanding (in years)	7 years 21 days
Weighted Average Exercise Price of options outstanding (in dollars per share)	$ 37.47
Number Exercisable (in shares) | shares	608,320
Weighted Average Exercise Price of options exercisable (in dollars per share)	$ 34.33