Stockholders' Equity - Sale of Common Stock (Details) - USD ($)	1 Months Ended			2 Months Ended
	Dec. 31, 2017	Oct. 31, 2017	May 31, 2016	Sep. 30, 2017
Public Stock Offering [Member]
Class of Stock [Line Items]
Aggregate offering price	$ 75,000,000.0		$ 25,000,000.0
Stock issued (shares)		7,700,000		1,550,880
Net proceeds from stock issuance		$ 271,300,000		$ 19,400,000
Fixed commission rate (percent)			3.00%
Stock offer price (in usd per share)		$ 37.50
Over-Allotment Option [Member]
Class of Stock [Line Items]
Stock issued (shares)		1,000,000