Stockholders' Equity - Summary of Number of PSUs granted and Assumptions Used to Calculate the Grant Date Fair Value Based on Total Shareholder Return (Detail) - PSUs – Total Shareholder Return - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of units granted (in shares)	100,958	124,755	65,796
Fair value on grant date (in usd per share)	$ 44.24	$ 54.94	$ 59.91
Expected price volatility, percent	25.31%	26.69%	29.90%
Risk-free interest rate	1.56%	0.98%	1.05%