Summary of Significant Accounting Policies (Policies)	6 Months Ended
Summary of Significant Accounting Policies (Policies)	Jun. 30, 2018
Summary of Significant Accounting Policies
Principles of Consolidation	Principles of Consolidation The accompanying unaudited interim consolidated financial statements include the financial statements of the Company and its wholly-owned subsidiaries. All significant intercompany balances and transactions have been eliminated in consolidation.
Interim Financial Statements	Interim Financial Statements The accompanying unaudited interim consolidated financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission (SEC). In the opinion of management, all adjustments, which consist solely of normal recurring adjustments, necessary to present fairly, in accordance with U.S. generally accepted accounting principles (GAAP), the financial position, results of operations and cash flows for all periods presented, have been made. The results of operations for the interim periods presented are not necessarily indicative of the results that may be expected for the full year. Certain information and footnote disclosures normally included in annual consolidated financial statements prepared in accordance with GAAP have been condensed or omitted. These unaudited interim consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and notes thereto included in the Company’s Annual Report on Form 10-K, filed for the fiscal year ended December 31, 2017. The information presented in the accompanying unaudited interim consolidated balance sheet as of December 31, 2017, has been derived from the Company’s December 31, 2017 audited consolidated financial statements. All other information has been derived from the unaudited interim consolidated financial statements of the Company.
Revenue Recognition	Revenue Recognition The Company enters into contracts that may contain one or a combination of fuel cell systems and infrastructure, installation, maintenance, spare parts, fuel delivery and other support services. Contracts containing fuel cell systems and related infrastructure may be sold, or provided to customers under a PPA. The Company does not include a right of return on its products other than rights related to standard warranty provisions that permit repair or replacement of defective goods. The Company accrues for anticipated standard warranty costs at the same time that revenue is recognized for the related product, or when circumstances indicate that warranty costs will be incurred, as applicable. Only a limited number of fuel cell units are under standard warranty. Revenue is measured based on the consideration specified in a contract with a customer, subject to the allocation of consideration to individual performance obligations as discussed below. The Company recognizes revenue when it satisfies a performance obligation by transferring control over a product or service to a customer. The Company accounts for each separate performance obligation of multiple deliverable arrangements as a separate unit of accounting if the delivered item or items have value to the customer on a standalone basis. The Company considers a performance obligation to be distinct and have a standalone value if the customer can benefit from the good or service either on its own or together with other resources readily available to the customer and the Company’s promise to transfer the goods or service to the customer is separately identifiable from other promises in the contract. The Company allocates revenue to each separate performance obligation based on relative standalone selling prices. Payment terms for fuel cells, infrastructure and service are invoiced with terms ranging from 30 to 90 days. Service is prepaid upfront in a majority of the arrangements. The Company does not adjust the transaction price for a significant financing component when the performance obligation is expected to be fulfilled within a year. Nature of goods and services The following is a description of principal activities from which the Company generates its revenue. (i)Sales of Fuel Cell Systems and Related Infrastructure Revenue from sales of fuel cell systems and related infrastructure represents sales of our GenDrive units, GenSure stationary backup power units, as well as hydrogen fueling infrastructure. The Company considers list prices, as well as historical average pricing approaches to determine standalone selling prices. Once relative standalone selling prices are determined, the Company proportionately allocates the sale consideration to each performance obligation within the customer arrangement. The allocated sales consideration related to fuel cell systems and infrastructure, spare parts, and hydrogen infrastructure is recognized at a point in time, when the performance obligation has been satisfied, which usually occurs at shipment if title and risk of loss have passed to the customer or upon commissioning. (ii)Services performed on fuel cell systems and related infrastructure Revenue from services performed on fuel cell systems and related infrastructure represents revenue earned on our service and maintenance contracts and sales of spare parts. The sales consideration allocated to services as discussed above is generally recognized as revenue over time on a straight-line basis over the expected service period. In a vast majority of its commercial transactions, the Company sells extended maintenance contracts that generally provide for a five to ten year service period from the date of product installation. Services include monitoring, technical support, maintenance and services that provide for 97-98% uptime of the fleet. These services are accounted for as a separate performance obligation, and accordingly, revenue generated from these transactions, subject to the proportional allocation of sale consideration, is deferred and recognized in income over the term of the contract, generally on a straight-line basis. Additionally, the Company may enter into annual service and extended maintenance contracts that are billed monthly. Revenue generated from these transactions is recognized in income on a straight-line basis over the term of the contract. Costs are recognized as incurred over the term of the contract. Sales of spare parts are included within service revenue on the accompanying consolidated statement of operations. When costs are projected to exceed revenues over the life of the contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience and consider the estimated impact of the Company’s cost reduction initiatives. The actual results may differ from these estimates. When costs are projected to exceed revenues over the life of an extended maintenance contract, an accrual for loss contracts is recorded. Costs are estimated based upon historical experience and consider the estimated impact of the Company’s cost reduction initiatives. The actual results may differ from these estimates. Upon expiration of the extended maintenance contracts, customers either choose to extend the contract or switch to purchasing spare parts and maintaining the fuel cell systems on their own. (iii)Power Purchase Agreements Revenue from PPAs primarily represents payments received from customers who make monthly payments to access the Company’s GenKey solution. When fuel cell systems and related infrastructure are provided to customers through a PPA, revenues associated with these agreements are treated as rental income and recognized on a straight-line basis over the life of the agreements. In conjunction with entering into a PPA with a customer, the Company may enter into sale/leaseback transactions with third-party financial institutions, whereby the fuel cells, a majority of the related infrastructure, and, in some cases service are sold to the third-party financial institution and leased back to the Company through either an operating or capital lease. Certain of the Company’s sale/leaseback transactions with third-party financial institutions are required to be accounted for as capital leases. As a result, no upfront revenue was recognized at the closing of these transactions and a finance obligation for each lease was established. The fuel cell systems and related infrastructure that are provided to customers through these PPAs are considered leased property on the accompanying unaudited interim consolidated balance sheet. Costs to service the leased property, depreciation of the leased property, and other related costs are considered cost of PPA revenue on the accompanying unaudited interim consolidated statement of operations. Interest cost associated with capital leases is presented within interest and other expense, net on the accompanying unaudited interim consolidated statement of operations. Each PPA entered into before December 31, 2015 had a corresponding sale/leaseback transaction with a third-party financial institution, which was required to be accounted for as an operating lease. The Company has rental expense associated with these sale/leaseback agreements with financial institutions. Rental expense is recognized on a straight-line basis over the life of the agreements and is characterized as cost of PPA revenue on the accompanying unaudited interim consolidated statement of operations. (iv)Fuel Delivered to Customers Revenue associated with fuel delivered to customers represents the sale of hydrogen to customers that has been purchased by the Company from a third party or generated on site. The Company purchases hydrogen fuel from suppliers and sells to its customers upon delivery. Revenue and cost of revenue related to this fuel is recorded as dispensed, and is included in the respective “Fuel delivered to customers” lines on the unaudited interim consolidated statements of operations. (v)Other Other revenue primarily represents cost reimbursement research and development contracts associated with the development of PEM fuel cell technology. Contract accounting is used for research and development contract revenue. The Company generally shares in the cost of these programs with cost sharing percentages ranging from 30% to 50% of total project costs. Revenue from time and material contracts is recognized on the basis of hours expended plus other reimbursable contract costs incurred during the period and is included within the “other” revenue line on the unaudited interim consolidated statement of operations. All allowable work performed through the end of each calendar quarter is billed, subject to limitations in the respective contracts. Contract costs The Company expects that incremental commission fees paid to employees as a result of obtaining sales contracts are recoverable and therefore the Company capitalizes them as contract costs. Capitalized commission fees are amortized on a straight line basis over the period of time over which the transfer of goods or services to which the assets relate occur, typically ranging from 5 to 10 years. Amortization of the capitalized commission fees is included in selling, general, and administrative expenses. The Company recognizes the incremental costs of obtaining contracts as an expense when incurred if the amortization period of the assets that the Company otherwise would have recognized is one year or less. These costs are included in selling, general, and administrative expenses. During 2017, the Company issued warrants to Amazon.com, Inc. and Wal-mart Stores, Inc. The fair value of warrants associated with each of these transactions are accounted for as revenue incentives as described in Note 11, Warrant Transaction Agreements. Adoption of ASC Topic 606 - Transition Approach As discussed in Recent Accounting Pronouncements, on January 1, 2018, the Company adopted ASC Topic 606, Revenue from Contracts with Customers (ASC Topic 606), which offers two transition approaches: full retrospective and modified retrospective. The Company chose the modified retrospective approach as its transition method and will not experience a significant effect on the timing and amount of revenue recognized or the amount of revenue allocated to the identified performance obligations. There was an insignificant amount of historical contract acquisition costs that were expensed and were not capitalized upon adoption of ASC Topic 606. However, upon adoption, contract acquisition costs of $0.1 million were capitalized and are being amortized as described above.
Cash Equivalents	Cash Equivalents Cash equivalents consist of money market accounts with an initial term of less than three months. At June 30, 2018 and December 31, 2017, cash equivalents consist of money market accounts. For purposes of the unaudited interim consolidated statements of cash flows, the Company considers all highly-liquid debt instruments with original maturities of three months or less to be cash equivalents. The Company’s cash and cash equivalents are deposited with financial institutions located in the U.S. and may at times exceed insured limits.
Common Stock Warrant Accounting	Common Stock Warrant Accounting The Company accounts for common stock warrants as either derivative liabilities or as equity instruments depending on the specific terms of the warrant agreement. Derivative Liabilities Registered common stock warrants that require the issuance of registered shares upon exercise and do not sufficiently preclude an implied right to cash settlement are accounted for as derivative liabilities. We currently classify these derivative warrant liabilities on the accompanying unaudited interim consolidated balance sheets as a long-term liability, which are revalued at each balance sheet date subsequent to the initial issuance, using the Black-Scholes pricing model. This pricing model, which is based, in part, upon unobservable inputs for which there is little or no market data, requires the Company to develop its own assumptions. Changes in the fair value of the warrants are reflected in the accompanying unaudited interim consolidated statements of operations as change in fair value of common stock warrant liability. Equity Instruments Common stock warrants that meet certain applicable requirements of ASC Topic 815-40, Derivatives and Hedging – Contracts in Entity’s Own Equity, and other related guidance, including the ability of the Company to settle the warrants without the issuance of registered shares or the absence of rights of the grantee to require cash settlement, are accounted for as equity instruments. The Company classifies these equity instruments within additional paid-in capital on the accompanying unaudited interim consolidated balance sheets. Common stock warrants accounted for as equity instruments represent the warrants issued to Amazon.com, Inc. and Wal-Mart Stores, Inc. as discussed in Note 11. These warrants are remeasured at each financial reporting date prior to vesting, using the Monte Carlo pricing model. Once these warrants vest, they are no longer remeasured. This pricing model, which is based, in part, upon unobservable inputs for which there is little or no market data, requires the Company to develop its own assumptions. Changes in fair value resulting from remeasurement of common stock warrants issued in connection with the Amazon Transaction Agreement and the Walmart Transaction Agreement, as described in Note 11, Warrant Transaction Agreements, and are recorded as cumulative catch up adjustments as a reduction of revenue.
Convertible Senior Notes	Convertible Senior Notes The Company accounts for the issued Convertible Senior Notes with separate liability and equity components. The carrying amount of the liability component was initially determined by estimating the fair value of a similar debt instrument that does not have an associated convertible feature. The carrying amount of the equity component representing the conversion option was determined by deducting the estimated fair value of the liability component from the par value of the Convertible Senior Notes as a whole as of the date of issuance. This difference represents a debt discount that is amortized to interest expense, with a corresponding increase to the carrying amount of the liability component, over the term of the Convertible Senior Notes using the effective interest rate method. The equity component is not remeasured as long as it continues to meet the conditions for equity classification. The Company has allocated issuance costs incurred to the liability and equity components. Issuance costs attributable to the liability component are being amortized to expense over the respective term of the Convertible Senior Notes, and issuance costs attributable to the equity components were netted with the respective equity component in additional paid-in capital.
Use of Estimates	Use of Estimates The unaudited interim consolidated financial statements of the Company have been prepared in conformity with U.S. generally accepted accounting principles, which require management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the unaudited interim consolidated financial statements and the reported amounts of revenues and expenses during the reporting period. Actual results could differ from those estimates.
Reclassifications	Reclassifications Reclassifications are made, whenever necessary, to prior period financial statements to conform to the current period presentation. These reclassifications did not have a net impact on the results of operations or net cash flows in the periods presented.
Recent Accounting Pronouncements	Recent Accounting Pronouncements Recently Adopted Accounting Pronouncements In June 2014, an accounting update was issued that replaces the existing revenue recognition framework regarding contracts with customers. The Company adopted this accounting update as of January 1, 2018. The standard outlines a five-step model whereby revenue is recognized as performance obligations within a contract are satisfied. The standard also requires new, expanded disclosures regarding revenue recognition. The Company did not experience a significant effect on the timing and amount of revenue recognized or the amount of revenue allocated to the identified performance obligations. There is an insignificant amount of historical contract acquisition costs that were expensed under prior guidance and were not capitalized upon adoption of ASC Topic 606. However, in subsequent periods, contract acquisition costs are capitalized in accordance with ASC Topic 606 (see Note 12, Revenue). In October 2016, an accounting update was issued to simplify how an entity should recognize the income tax consequences of an intra-entity transfer of an asset other than inventory when the transfer occurs. Consequently, the amendments in this update eliminate the exception for an intra-entity transfer of an asset other than inventory. Two common examples of assets included in the scope of this update are intellectual property and property, plant, and equipment. The Company adopted this update on January 1, 2018 and it did not have any effect on the consolidated financial statements because our net tax position is zero. In November 2016, an accounting update was issued to reduce the existing diversity in the classification and presentation of changes in restricted cash on the statement of cash flows. This accounting update was adopted retrospectively by the Company on January 1, 2018. The adoption of this update impacts the cash flows from financing activities due to the change in the presentation of restricted cash within the consolidated statement of cash flows. Net cash flows from financing activities and change in cash and cash equivalents, which now includes restricted cash, for the six months ended June 30, 2018 and 2017, decreased by $2.8 million and $2.6 million, respectively. Recently Issued and Not Yet Adopted Accounting Pronouncements In June 2018, an accounting update was issued to simplify the accounting for nonemployee share-based payment transactions resulting from expanding the scope of ASC Topic 718, Compensation-Stock Compensation, to include share-based payment transactions for acquiring goods and services from nonemployees. An entity should apply the requirements of ASC Topic 718 to nonemployee awards except for specific guidance on inputs to an option pricing model and the attribution of cost (that is, the period of time over which share-based payment awards vest and the pattern of cost recognition over that period). The amendments specify that ASC Topic 718 applies to all share-based payment transactions in which a grantor acquires goods or services to be used or consumed in a grantor’s own operations by issuing share-based payment awards. The amendments also clarify that ASC Topic 718 does not apply to share-based payments used to effectively provide (1) financing to the issuer or (2) awards granted in conjunction with selling goods or services to customers as part of a contract accounted for under ASC Topic 606, Revenue from Contracts with Customers. The amendments in this accounting update are effective for public business entities for fiscal years beginning after December 15, 2018, including interim periods within that fiscal year. Early adoption is permitted, but no earlier than an entity’s adoption date of ASC Topic 606. The Company is evaluating whether to early adopt this accounting update during the remainder of 2018. In February 2016, an accounting update was issued which requires balance sheet recognition for operating leases, among other changes to previous lease guidance. This accounting update is effective for fiscal years beginning after December 15, 2018. The Company has established an internal implementation team to oversee the adoption of the new standard, and has estimated the impact this accounting update will have on its consolidated financial statements. When adopted, the Company expects there to be an increase in finance obligations of over $30.0 million, which represents the future minimum lease payments under non-cancelable operating leases, as lessee, and a corresponding addition of a right-of-use asset. Any cumulative effect of the adoption recorded to accumulated deficit is not expected to be significant. The Company also does not expect there to be a significant net effect on its consolidated statements of operations in the current or prior periods, however what was previously presented as rent expense related to operating leases will be recognized as interest expense on the Company’s minimum lease obligation and depreciation of its right-of-use asset. In July 2018, an accounting update was issued to make technical amendments to the previous update, including the addition of a new optional transition method. The Company is evaluating whether to early adopt this accounting update during the remainder of 2018. The Company is also evaluating the transition method of adoption. In January 2017, an accounting update was issued to simplify how an entity is required to test goodwill for impairment by eliminating Step 2 from the goodwill impairment test. Step 2 measures a goodwill impairment loss by comparing the implied fair value of a reporting unit’s goodwill with the carrying amount of that goodwill. This accounting update is effective for years beginning after December 15, 2019. Early adoption is permitted for interim or annual goodwill impairment tests performed on testing dates after January 1, 2017. The Company is evaluating the impact this update will have on the consolidated financial statements. In August 2016, an accounting update was issued to reduce the existing diversity in practice in how certain cash receipts and cash payments are presented and classified in the statement of cash flows. This accounting update is effective for fiscal years beginning after December 15, 2018, and interim periods within fiscal years beginning after December 15, 2019. Early adoption is permitted, including adoption in an interim period. The Company is evaluating the impact this update will have on the consolidated financial statements.