Equity - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions		1 Months Ended
	Dec. 15, 2017	Jul. 31, 2018	Jan. 31, 2018
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering	3.2		0.3
Shares issued, price per share	$ 185.00		$ 185.00
Proceeds from public offering of common stock, net of issuance costs	$ 569.8		$ 48.7
Inclusive of Shares Pursuant to Over Allotment Option Granted [Member]
Subsidiary Sale Of Stock [Line Items]
Number of shares issued in public offering		3.9
Shares issued, price per share		$ 162.50
Proceeds from public offering of common stock, net of issuance costs		$ 600.6