Equity Financing - Additional Information (Detail) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended
	Jul. 31, 2017	Sep. 30, 2016	Jun. 30, 2016	Oct. 31, 2015
Class Of Stock [Line Items]
Common stock issued		5.8	2.1	3.3
Common stock average price per share	$ 42.50	$ 59.75	$ 17.84	$ 39.00
Net proceeds from issue of common stock	$ 354.0	$ 327.4	$ 37.3	$ 119.9
Underwriting discounts, commission and offering expenses	$ 20.0	$ 17.6	$ 0.2	$ 6.8
Underwriting discount and commission per share			$ 1.60
Underwritten Public Offering [Member]
Class Of Stock [Line Items]
Common stock issued	8.8
Underwritten Offering [Member]
Class Of Stock [Line Items]
Common stock issued	1.2