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Stock-Based Compensation - Narrative (Details) |  |  |  | 1 Months Ended | 3 Months Ended | 6 Months Ended | 9 Months Ended | 12 Months Ended |  |
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Dec. 03, 2015 $ / shares shares | May 29, 2015 $ / shares | Dec. 09, 2014 tranche $ / shares shares | Dec. 31, 2012 | Mar. 31, 2012 shares | Jun. 30, 2018 $ / shares shares | Jun. 30, 2017 $ / shares shares | Jun. 30, 2017 $ / shares shares | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | Dec. 31, 2015 USD ($) $ / shares shares | Jun. 21, 2017 USD ($) | Dec. 05, 2012 tranche
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted (in shares) |  |  |  |  |  | 1,179,877 | 2,183,106 | 4,015,356 |  | 4,386,856 |  |  |  |
Weighted average exercise price (in dollars per share) | $ / shares |  |  |  |  |  | $ 0.96 | $ 1.46 | $ 1.51 |  | $ 1.51 |  |  |  |
Weighted average period for recognition of compensation cost related to unvested stock awards |  |  |  |  |  |  |  |  |  | 2 years 7 months 17 days |  |  |  |
Total unrecognized compensation cost, net of estimated forfeitures, related to non-vested stock options and restricted stock | $ |  |  |  |  |  |  |  |  |  | $ 4,200,000 |  |  |  |
RSU's granted (in dollars per share) | $ / shares |  |  |  |  |  | $ 0.70 | $ 1.11 | $ 1.15 |  |  |  |  |  |
Non-employees |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted (in shares) |  |  |  |  |  |  | 150,000 | 150,000 |  |  |  |  |  |
Weighted average exercise price (in dollars per share) | $ / shares |  |  |  |  |  |  | $ 1.60 | $ 1.60 |  |  |  |  |  |
Founders' |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vested (in shares) |  |  |  |  |  |  |  |  |  |  |  | 329,021 |  |
Allocated share-based compensation expense | $ |  |  |  |  |  |  |  |  |  | 0 | $ 0 | $ 3,400,000 |  |
Stock options |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated share-based compensation expense | $ |  |  |  |  |  |  |  |  |  | $ 6,600,000 | 8,700,000 | 14,100,000 |  |
Total intrinsic value | $ |  |  |  |  |  |  |  |  |  |  | $ 300,000 | $ 6,700,000 |  |
Options granted (in shares) |  |  |  |  |  |  |  |  |  | 4,236,856 | 2,570,600 | 1,744,600 |  |
Weighted average exercise price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 1.51 | $ 6.80 | $ 33.13 |  |
Weighted average grant date fair value of options granted (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 1.14 | $ 4.84 | $ 20.87 |  |
Stock options | Non-employees |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated share-based compensation expense | $ |  |  |  |  |  |  |  |  |  | $ 300,000 | $ 100,000 | $ 1,100,000 |  |
Options granted (in shares) |  |  |  |  |  |  |  |  |  | 150,000 | 10,000 | 50,500 |  |
Weighted average exercise price (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 1.60 | $ 6.96 | $ 36.25 |  |
Stock Compensation Plan [Member] | President [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Weighted average period for recognition of compensation cost related to unvested stock awards |  |  |  |  |  |  |  |  |  | 2 years |  |  |  |
Stock-based compensation expense | $ |  |  |  |  |  |  |  |  | $ 100,000 |  |  |  |  |
Total unrecognized compensation cost, net of estimated forfeitures, related to non-vested stock options and restricted stock | $ |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,700,000 |
Restricted stock units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vested (in shares) |  |  |  |  |  |  |  |  |  | 0 |  |  |  |
Options granted (in shares) |  |  |  |  |  |  |  |  |  | 0 |  |  |  |
RSU's granted (in dollars per share) | $ / shares |  |  |  |  |  |  |  |  |  | $ 0.00 |  |  |  |
Total fair value of RSUs that vested | $ |  |  |  |  |  |  |  |  |  | $ 0 | $ 100,000 | $ 500,000 |  |
Restricted stock units | Tranche one |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted (in shares) | 75,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting percentage | 25.00% |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock units | Tranche two |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted (in shares) | 10,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock units | Chief Executive Officer | December 2014 Award |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated share-based compensation expense | $ |  |  |  |  |  |  |  |  |  |  |  | 700,000 |  |
Weighted average exercise price (in dollars per share) | $ / shares |  | $ 43.47 |  |  |  |  |  |  |  |  |  |  |  |
Performance criteria |  |  | 150.00% |  |  |  |  |  |  |  |  |  |  |
Restricted stock units | Chief Executive Officer | Service-based restricted stock units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vesting period (in years) |  |  | 24 months | 24 months |  |  |  |  |  |  |  |  |  |
Restricted stock units | Chief Executive Officer | Service-based restricted stock units | December 2014 Award |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vested (in shares) |  |  | 15,450 |  |  |  |  |  |  |  |  |  |  |
Allocated share-based compensation expense | $ |  |  |  |  |  |  |  |  |  |  |  | 500,000 |  |
Options granted (in shares) |  |  | 30,902 |  |  |  |  |  |  |  |  |  |  |
RSU's granted (in dollars per share) | $ / shares |  |  | $ 32.36 |  |  |  |  |  |  |  |  |  |  |
Restricted stock units | Chief Executive Officer | Performance-based restricted stock units |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of tranches | tranche |  |  | 2 |  |  |  |  |  |  |  |  |  |  | 2
Vesting period (in years) |  |  | 1 year | 1 year |  |  |  |  |  |  |  |  |  |
Restricted stock units | Chief Executive Officer | Performance-based restricted stock units | December 2014 Award |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated share-based compensation expense | $ |  |  |  |  |  |  |  |  |  |  |  | $ 200,000 |  |
Restricted stock units | Senior executives and a non-employee consultant |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Allocated share-based compensation expense | $ |  |  |  |  |  |  |  |  |  |  | $ 300,000 |  |  |
Options granted (in shares) | 85,000 |  |  |  |  |  |  |  |  |  |  |  |  |
RSU's granted (in dollars per share) | $ / shares | $ 9.81 |  |  |  |  |  |  |  |  |  |  |  |  |
Restricted stock units | Senior executives and a non-employee consultant | Tranche two |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Vested (in shares) |  |  |  |  |  |  |  |  |  |  | 16,250 |  |  |
Options granted (in shares) |  |  |  |  |  |  |  |  |  |  | 85,000 |  |  |
Vesting period (in years) | 12 months |  |  |  |  |  |  |  |  |  |  |  |  |
Forfeitures (in shares) |  |  |  |  |  |  |  |  |  |  | 68,750 |  |  |
2012 Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares available for future issuance (in shares) |  |  |  |  | 1,453,253 |  |  |  |  |  |  |  |  |
2012 Plan | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Annual increase to the lowest number of shares of common stock (in shares) |  |  |  |  | 975,000 |  |  |  |  |  |  |  |  |
Annual increase as a percentage of the number of shares of common stock outstanding |  |  |  |  | 4.00% |  |  |  |  |  |  |  |  |
2011 Plan | Maximum |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Shares available for future issuance (in shares) |  |  |  |  | 679,622 |  |  |  |  |  |  |  |  |

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- Definition Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB Topic 14.F) -URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5047-113901 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(1)(i) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Fair value of share-based awards for which the grantee gained the right by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition Number of shares under non-option equity instrument agreements that were cancelled as a result of occurrence of a terminating event. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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- Definition An excess of the fair value of the modified award over the fair value of the award immediately before the modification. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (h)(2)(iii) -URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901 + Details
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