Debt Obligations - Additional Information (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Jul. 15, 2016	Jul. 25, 2011
Aggregate principal amount			$ 60,000	$ 60,000
Debt instrument interest rate			3.75%	3.75%
Issuance of shares of common stock on conversion		540
Warrant or right to purchase shares of common stock	386			4,634
Warrant or right to purchase shares of common stock, strike price	$ 17.00
Exercised warrants settled in shares	102	4,248
Exercised warrants payable in cash		$ 10,525
Exercised warrants paid in cash	$ 1,025	$ 9,500
Weighted average stock price	$ 23.13	$ 19.82
Common Stock [Member]
Exercised warrants settled in shares		80