Common Stock and Other Equity Related Transactions - At-market offering (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Par or stated value per share (usd per share)	$ 0.01	$ 0.01
Shares of Common Stock sold (in shares)	861,141	1,380,017	683,548
Commissions paid to sales agents	$ 1,651	$ 1,947	$ 1,122
Private Placement
Class of Stock [Line Items]
Total gross proceeds	78,755	$ 120,698	$ 50,000
Private Placement | Common Stock
Class of Stock [Line Items]
Aggregate offering price	$ 200,000
Shares of Common Stock sold (in shares)	861,141	1,380,017	683,548
Weighted average price (in usd per share)	$ 91.45	$ 87.46	$ 73.15
Commissions paid to sales agents	$ 1,028	$ 1,512	$ 657