Significant Accounting Policies - Summary of Assumptions for Options Granted (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Volatility factor	14.80%	19.20%	23.60%
Dividend yield	0.70%	1.10%	0.90%
Expected term (in years)	5 years 8 months 16 days	5 years 9 months 11 days	5 years 7 months 28 days
Risk-free rate	2.00%	1.30%	1.60%