Stockholders' Equity - Warrants Issued to USAA and Third Party Marketing Firm (Details) $ / shares in Units, $ in Thousands			1 Months Ended	12 Months Ended
	May 01, 2014 tranche $ / shares shares	Feb. 25, 2011 $ / shares shares	Mar. 31, 2015 shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($) shares
Convertible Preferred Stock and Stockholders' Equity
Warrant expense | $				$ 0	$ 46	$ (803)
Common Stock Purchase Warrants | Third Party Marketing Firm
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement		1,433,333	1,433,333
Exercise price of warrants (in dollars per share) | $ / shares		$ 6.02
Period of warrant		8 years
Shares issued for exercise of warrants			959,676
USAA
Convertible Preferred Stock and Stockholders' Equity
Warrants earned and outstanding (in shares)				509,642
Remaining warrants available for issuance (in shares)				949,337
USAA | Common Stock Purchase Warrants
Convertible Preferred Stock and Stockholders' Equity
Warrant expense | $				$ 0	$ 100	$ 200
Number of shares under warrants that have been earned					10,666	71,330
USAA | Common Stock Purchase Warrants | Affinity Group Marketing Agreement
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement	1,458,979
Number of warrant tranches | tranche	2
USAA | Common Stock Purchase Warrants | Affinity Group Marketing Agreement | Tranche One
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement	392,313
Exercise price of warrants (in dollars per share) | $ / shares	$ 7.95
USAA | Common Stock Purchase Warrants | Affinity Group Marketing Agreement | Tranche Two
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement	1,066,666
Exercise price of warrants (in dollars per share) | $ / shares	$ 15.00