Common Stock and Other Equity Related Transactions - At-market offering (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 02, 2017	Nov. 01, 2017
Class of Stock [Line Items]
Par or stated value per share (usd per share)	$ 0.01	$ 0.01
Common stock issued through the At-The-Market Offering Program (in shares)	1,380,017	683,548	0
Commissions paid to sales agents	$ 1,947	$ 1,122	$ 491
Private Placement
Class of Stock [Line Items]
Aggregate offering price				$ 200,000	$ 125,000
Total gross proceeds	121	$ 50
Common stock, available for issuance	$ 150,000
Private Placement | Common Stock
Class of Stock [Line Items]
Common stock issued through the At-The-Market Offering Program (in shares)	1,380,017	683,548
Weighted average price (in usd per share)	$ 87.46	$ 73.15
Commissions paid to sales agents	$ 1,512	$ 657