Stockholders' Equity - Additional Information (Detail)		3 Months Ended	9 Months Ended
	May 06, 2016 USD ($) sales_agent	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)
Class of Stock [Line Items]
Proceeds from the sale of common stock, net of offering costs			$ 33,780,000	$ 0
At the Market Equity Distribution Agreement
Class of Stock [Line Items]
Aggregate offering price of common share	$ 400,000,000
Number of sales agents | sales_agent	5
Shares, issued (in shares) | shares		343,251	343,251
Average price per share (in dollars per share) | $ / shares		$ 99.75	$ 99.75
Proceeds from the sale of common stock, net of offering costs		$ 33,780,000	$ 33,780,000
Value of stock available for issuance under ATM		$ 315,135,000	$ 315,135,000