Stockholders' Equity - Sale of Common Stock (Details) - Public Stock Offering [Member] - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		2 Months Ended
	Aug. 31, 2018	Oct. 31, 2017	Sep. 30, 2017
Class of Stock [Line Items]
Stock issued (shares)	6,000,000	7,700,000	1,550,880
Net proceeds from stock issuance	$ 292.9	$ 271.3	$ 19.4
Stock offer price (in usd per share)	$ 52.00	$ 37.50