Equity Financing - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended
	Nov. 30, 2018	Jul. 31, 2017	Sep. 30, 2016	Jun. 30, 2016
Class Of Stock [Line Items]
Common stock issued			5.8	2.1
Common stock average price per share	$ 131.00	$ 42.50	$ 59.75	$ 17.84
Net proceeds from issue of common stock	$ 513.4	$ 354.0	$ 327.4	$ 37.3
Underwriting discounts, commission and offering expenses	$ 24.6	$ 20.0	$ 17.6	$ 0.2
Underwritten Public Offering [Member]
Class Of Stock [Line Items]
Common stock issued	4.1	8.8
Underwritten Offering [Member]
Class Of Stock [Line Items]
Common stock issued	0.3	1.2