Stockholders' Equity (Details 2) (USD $)	12 Months Ended			1 Months Ended		0 Months Ended	1 Months Ended	0 Months Ended						1 Months Ended	0 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Nov. 30, 2013	Aug. 29, 2013	Jun. 30, 2014	Jun. 13, 2012	Jun. 29, 2012	Apr. 12, 2012	Dec. 31, 2012	Oct. 01, 2011	Feb. 25, 2011	May 31, 2014	Jun. 25, 2010	Jan. 01, 2012	May 01, 2014	Jun. 13, 2013	Apr. 29, 2011	Nov. 24, 2009	Dec. 31, 2011
Convertible Preferred Stock and Stockholders' Equity
Exercise of warrants	$ 9,461,000		$ 637,000
Proceeds from exercise of warrants	9,461,000		637,000
Warrant expense	9,861,000	3,740,000	1,990,000
Amount drawn on credit facility	5,000,000	5,000,000
Principal and accrued interest due amount of debt converted		25,447,000
Revolving line of credit
Convertible Preferred Stock and Stockholders' Equity
Amount drawn on credit facility				5,000,000
Vulcan
Convertible Preferred Stock and Stockholders' Equity
Aggregate purchase price for preferred stock and warrants					30,000,000
Common Stock Purchase Warrants | Revolving line of credit
Convertible Preferred Stock and Stockholders' Equity
Shares that may be purchased under warrant						66,666
Number of shares exercised through a net settlement							66,666
Exercise price of warrants (in dollars per share)						$ 7.92
Expected life		10 years
Risk-free interest rate (as a percent)		2.78%
Volatility (as a percent)		64.80%
Shares issued for exercise of warrants							27,526
Amount drawn on credit facility						5,000,000
Common Stock Purchase Warrants | Revolving line of credit | Draws on credit facility at any time after issuance date
Convertible Preferred Stock and Stockholders' Equity
Shares that may be purchased under warrant								26,666										26,666
Exercise price of warrants (in dollars per share)								$ 11.51										$ 7.92
Common Stock Purchase Warrants | Revolving line of credit | Advances greater than $4.0 million at any time after issuance date
Convertible Preferred Stock and Stockholders' Equity
Shares that may be purchased under warrant								66,666
Threshold borrowings under the credit facility for higher number of shares under warrant								4,000,000
Common Stock Purchase Warrants | Vulcan
Convertible Preferred Stock and Stockholders' Equity
Shares that may be purchased under warrant					666,666
Exercise price of warrants (in dollars per share)					$ 15.00
Expected life		2 years
Risk-free interest rate (as a percent)		0.31%
Volatility (as a percent)		49.40%
Aggregate purchase price for preferred stock and warrants					700,000
Common Stock Purchase Warrants | Yahoo Auto Buying Program | Yahoo
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement										8,000,000
Shares vesting in increments on quarterly basis										666,666
Exercise price of warrants (in dollars per share)										$ 11.51
Number of shares cancelled under unvested warrants									7,333,333
Warrant expense			100,000
Number of shares under warrants that have been earned			666,666						666,666
Volatility (as a percent)			50.90%
Common Stock Purchase Warrants | Yahoo Auto Buying Program | Yahoo | Minimum
Convertible Preferred Stock and Stockholders' Equity
Expected life			2 months 12 days
Risk-free interest rate (as a percent)			0.06%
Common Stock Purchase Warrants | Yahoo Auto Buying Program | Yahoo | Maximum
Convertible Preferred Stock and Stockholders' Equity
Expected life			2 years 10 months 24 days
Risk-free interest rate (as a percent)			0.50%
Common Stock Purchase Warrants | Honk LLC
Convertible Preferred Stock and Stockholders' Equity
Shares that may be purchased under warrant																			5,724
Exercise price of warrants (in dollars per share)																			$ 0.01
Common Stock Purchase Warrants | ALG, Inc.
Convertible Preferred Stock and Stockholders' Equity
Shares that may be purchased under warrant			4,231,416								4,231,416	4,231,416
Exercise price of warrants (in dollars per share)			$ 7.95								$ 7.95	$ 7.95
Expected life												1 year
Risk-free interest rate (as a percent)												0.19%
Volatility (as a percent)												45.00%
Shares issued for exercise of warrants											23,816
Common Stock Purchase Warrants | Third Party Marketing Firm
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement													1,433,333
Exercise price of warrants (in dollars per share)													$ 6.02
Period of warrant													8 years
Warrant expense	2,300,000	2,500,000	1,600,000
Number of shares under warrants that have been earned	343,665	604,266	207,710
Common Stock Purchase Warrants | Third Party Marketing Firm | Minimum
Convertible Preferred Stock and Stockholders' Equity
Expected life	4 years 9 months 18 days	5 years 2 months 12 days	5 years 9 months 18 days
Risk-free interest rate (as a percent)	1.51%	0.63%	0.62%
Volatility (as a percent)	46.60%	51.50%	58.60%
Common Stock Purchase Warrants | Third Party Marketing Firm | Maximum
Convertible Preferred Stock and Stockholders' Equity
Expected life	5 years 1 month 6 days	6 years 6 months	7 years 1 month 6 days
Risk-free interest rate (as a percent)	1.70%	1.81%	1.39%
Volatility (as a percent)	48.10%	59.50%	59.50%
Common Stock Purchase Warrants | Service Provider
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement														333,333
Exercise price of warrants (in dollars per share)														$ 12.81
Warrant vesting period														4 years
Period of warrant														5 years
Warrant expense	1,700,000
Number of shares under warrants that have been earned	33,333
Common Stock Purchase Warrants | Service Provider | Minimum
Convertible Preferred Stock and Stockholders' Equity
Expected life	4 years 4 months 24 days
Risk-free interest rate (as a percent)	1.48%
Volatility (as a percent)	54.80%
Common Stock Purchase Warrants | Service Provider | Maximum
Convertible Preferred Stock and Stockholders' Equity
Expected life	5 years
Risk-free interest rate (as a percent)	1.55%
Volatility (as a percent)	56.80%
Common Stock Purchase Warrants | USAA
Convertible Preferred Stock and Stockholders' Equity
Proceeds from exercise of warrants														9,500,000
Warrant expense	5,700,000	1,100,000	300,000
Number of shares under warrants that have been earned	631,449	415,349	31,201
Shares issued for exercise of warrants														3,265,168
Common Stock Purchase Warrants | USAA | Minimum
Convertible Preferred Stock and Stockholders' Equity
Expected life	3 months 18 days	2 years	7 years
Risk-free interest rate (as a percent)	0.02%	0.29%	1.02%
Volatility (as a percent)	48.50%	47.40%	50.00%
Common Stock Purchase Warrants | USAA | Maximum
Convertible Preferred Stock and Stockholders' Equity
Expected life	8 years	2 years 10 months 24 days	8 years
Risk-free interest rate (as a percent)	2.33%	0.52%	1.77%
Volatility (as a percent)	57.50%	52.80%	58.60%
Common Stock Purchase Warrants | USAA | Warrants agreement entered on March 12, 2009
Convertible Preferred Stock and Stockholders' Equity
Shares that may be purchased under warrant		961,482	961,482								961,482									961,482
Exercise price of warrants (in dollars per share)																				$ 0.83
Common Stock Purchase Warrants | USAA | Warrants agreement entered on June 25, 2010
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement															1,653,333
Shares that may be purchased under warrant																					1,653,333
Exercise price of warrants (in dollars per share)															$ 2.12
Common Stock Purchase Warrants | USAA | Warrants agreement entered on January 1, 2012
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement																1,042,666
Exercise price of warrants (in dollars per share)																$ 7.95
Period of warrant																8 years
Period of warrant after expiration of the affinity agreement																90 days
Common Stock Purchase Warrants | USAA | Affinity Group Marketing Agreement
Convertible Preferred Stock and Stockholders' Equity
Maximum number of shares under warrant agreement																	1,458,979
Number of warrant tranches																	2
Common Stock Purchase Warrants | USAA | 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)																	7.95
Common Stock Purchase Warrants | USAA | 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)																	15.00
Series A preferred stock | Vulcan
Convertible Preferred Stock and Stockholders' Equity
Aggregate purchase price for preferred stock and warrants					$ 29,200,000