Stockholders' Equity (Deficit) - Narrative (Details)			3 Months Ended		6 Months Ended	12 Months Ended
	Jan. 23, 2018 USD ($) $ / shares shares	Feb. 16, 2016 USD ($) $ / shares shares	Jun. 30, 2018 vote $ / shares shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) vote $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Nov. 13, 2013 USD ($)
Class of Stock [Line Items]
Shares issued in transaction (in shares)	4,665,594
Sale of stock price per share (in dollars per share) | $ / shares	$ 4.2867
Class of warrant or right, number of securities called by warrants or rights (in shares) (up to)	1,500,000
Warrant exercise price (in dollars per share) | $ / shares	$ 6.00
Proceeds from issuance of warrants | $	$ 20,000,000
Issuance of common stock and warrants | $					$ 19,300,000
Payments of stock issuance costs | $	700,000
Preferred stock, shares authorized (in shares)			5,000,000		5,000,000
Preferred stock, par or stated value per share (in dollars per share) | $ / shares			$ 0.01		$ 0.01
Investment warrants, exercise price (in dollars per share) | $ / shares		$ 3.50
Common stock, par value (in dollars per share) | $ / shares			$ 0.01		$ 0.01	$ 0.01
Common stock, vote per share | vote			1		1
Dividends declared or paid on common stock | $				$ 0	$ 0
Stock repurchase program, authorized amount | $							$ 50,000,000.0
Stock repurchased during period, shares (in shares)					0	855,474
Stock repurchased during period, value | $						$ 2,500,000
Treasury stock, shares retired (in shares)					0
Treasury stock, shares (in shares)			5,321,393		5,321,393	5,321,393
Restricted Stock Awards and Restricted Stock Units
Class of Stock [Line Items]
Shares paid for tax withholding for share-based compensation (in shares)			134,898		497,300
Redeemable Convertible Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued (in shares)			236,672		236,672	227,483
Preferred stock, dividend rate					8.00%	8.00%
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01	$ 0.01		$ 0.01	$ 0.01
Temporary equity, shares outstanding (in shares)			236,672		236,672	227,483
Redeemable Convertible Preferred Stock | TCP-ASC ACHI Series LLLP
Class of Stock [Line Items]
Payments of stock issuance costs | $		$ 21,300,000
Preferred stock, shares issued (in shares)		200,000
Preferred stock, dividend rate		8.00%			8.00%
Preferred stock, issued, value | $		$ 200,000,000
Long-term purchase commitment, period		10 years
Redeemable Convertible Preferred Stock | TCP-ASC ACHI Series LLLP | Investor
Class of Stock [Line Items]
Preferred stock, dividend rate		8.00%			8.00%
Common Stock | TCP-ASC ACHI Series LLLP
Class of Stock [Line Items]
Class of warrant or right, number of securities called by warrants or rights (in shares) (up to)		60,000,000
Investment warrants, exercise price (in dollars per share) | $ / shares		$ 3.50
Long-term purchase commitment, period		10 years
Additional Paid-In Capital
Class of Stock [Line Items]
Issuance of common stock and warrants | $	19,300,000				$ 19,300,000
Common Stock
Class of Stock [Line Items]
Issuance of common stock and warrants | $	$ 47,000