Issuance of Common Stock (Details) - USD ($) $ in Thousands		12 Months Ended
	Mar. 28, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Common stock shares issued (in shares)	5,175,000	50,248,000	44,645,000
Proceeds from issuance of common stock	$ 136,600	$ 143,810	$ 5,271	$ 6,668
Underwriting discounts and commissions	8,800
Other direct costs incurred	$ 816	$ 816	$ 0	$ 0