Subsequent Event - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Millions	Feb. 13, 2018	Nov. 17, 2017	Sep. 14, 2016	Jan. 12, 2016	Apr. 20, 2015
Subsequent Event [Line Items]
Issuance of common stock, shares		4,058,822	5,062,892	3,157,894	2,628,571
Common stock price per share		$ 85.00	$ 39.75	$ 47.50	$ 52.50
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering expenses payable		$ 325.8	$ 189.2	$ 140.4	$ 129.1
Subsequent Event [Member]
Subsequent Event [Line Items]
Issuance of common stock, shares	4,032,012
Common stock price per share	$ 164.00
Proceeds from public offering of common stock, net of commissions and underwriting discounts and estimated offering expenses payable	$ 631.1