iRhythm (IRTC) 10-K 4 Mar 19 2018 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Feb. 22, 2019	Jun. 29, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2,018
Document Fiscal Period Focus	FY
Trading Symbol	IRTC
Entity Registrant Name	iRhythm Technologies, Inc.
Entity Central Index Key	1,388,658
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		24,463,561
Entity Public Float			$ 1,743.2

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 20,023	$ 8,671
Short-term investments	58,320	93,692
Accounts receivable, net	21,977	12,953
Inventory	2,062	1,683
Prepaid expenses and other current assets	4,100	2,582
Total current assets	106,482	119,581
Investments, long-term		2,994
Property and equipment, net	9,158	6,221
Goodwill	862	862
Other assets	3,208	3,465
Total assets	119,710	133,123
Current liabilities:
Accounts payable	2,284	2,395
Accrued liabilities	26,570	15,644
Deferred revenue	1,243	1,238
Accrued interest, current portion	139	154
Debt, current portion		1,487
Total current liabilities	30,236	20,918
Debt	34,899	32,491
Deferred rent, noncurrent portion	153	161
Total liabilities	65,288	53,570
Commitments and contingencies (Note 6)
Stockholders’ equity:
Preferred stock, $0.001 par value – 5,000,000 shares authorized at December 31, 2018 and 2017; and none issued and outstanding at December 31, 2018 and 2017, respectively
Common stock, $0.001 par value – 100,000,000 shares authorized at December 31, 2018 and 2017; 24,368,073 and 23,377,315 shares issued and outstanding at December 31, 2018 and 2017, respectively	23	23
Additional paid-in capital	257,955	236,184
Accumulated other comprehensive loss	(41)	(65)
Accumulated deficit	(203,515)	(156,589)
Total stockholders’ equity	54,422	79,553
Total liabilities and stockholders’ equity	$ 119,710	$ 133,123

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Statement Of Financial Position [Abstract]
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	24,368,073	23,377,315
Common stock, shares outstanding	24,368,073	23,377,315

Consolidated Statements of Oper

Consolidated Statements of Operations - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Statement [Abstract]
Revenue	$ 147,293,000	$ 98,509,000	$ 64,072,000
Cost of revenue	38,579,000	27,708,000	20,883,000
Gross profit	108,714,000	70,801,000	43,189,000
Operating expenses:
Research and development	20,750,000	13,335,000	7,150,000
Selling, general and administrative	131,582,000	84,737,000	51,621,000
Total operating expenses	152,332,000	98,072,000	58,771,000
Loss from operations	(43,618,000)	(27,271,000)	(15,582,000)
Interest expense	(3,115,000)	(3,386,000)	(3,248,000)
Other income (expense), net	1,526,000	1,237,000	(2,073,000)
Loss on extinguishment of debt	(3,029,000)
Loss before income taxes	(48,236,000)	(29,420,000)	(20,903,000)
Income tax provision	44,000	0	0
Net loss	$ (48,280,000)	$ (29,420,000)	$ (20,903,000)
Net loss per common share, basic and diluted	$ (2.02)	$ (1.30)	$ (3.95)
Weighted-average shares, basic and diluted	23,885,858	22,627,327	5,285,847

Consolidated Statements of Comp

Consolidated Statements of Comprehensive Loss - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement Of Income And Comprehensive Income [Abstract]
Net Loss	$ (48,280)	$ (29,420)	$ (20,903)
Other comprehensive income (loss):
Net change in unrealized gains (losses) on available-for-sale securities	24	(56)	(9)
Comprehensive loss	$ (48,256)	$ (29,476)	$ (20,912)

Consolidated Statements of Conv

Consolidated Statements of Convertible Preferred Stock and Stockholders' Equity (Deficit) - USD ($) $ in Thousands	Total	Initial Public Offering	Convertible Preferred Stock	Convertible Preferred StockInitial Public Offering	Common Stock	Common StockInitial Public Offering	Additional Paid-In Capital	Additional Paid-In CapitalInitial Public Offering	Accumulated Deficit	Accumulated Other Comprehensive Loss
Balance at Dec. 31, 2015	$ (101,624)				$ 1		$ 4,641		$ (106,266)
Balance, Shares at Dec. 31, 2015					1,410,565
Balance, Convertible Preferred Stock at Dec. 31, 2015			$ 97,096
Balance, Convertible Preferred Stock, Shares at Dec. 31, 2015			11,046,146
Issuance of common stock upon the exercise of options	$ 132				$ 1		131
Issuance of common stock upon the exercise of options, Shares	57,067				56,827
Issuance of preferred stock upon exercise of warrants			$ 457
Issuance of preferred stock upon exercise of warrants, Shares			31,359
Issuance of common stock in connection with initial public offering, net of offering costs	$ 110,721				$ 7		110,714
Issuance of common stock in connection with initial public offering, net of offering costs, Shares					7,238,235
Conversion of convertible preferred stock to common stock in connection with initial public offering		$ 97,553		$ (97,553)		$ 13		$ 97,540
Conversion of convertible preferred stock to common stock in connection with initial public offering, Shares				(11,077,505)		13,375,333
Conversion of convertible preferred stock warrants to common stock warrants in connection with initial public offering	4,821						4,821
Issuance of common stock upon net exercise of warrants, Shares					58,386
Stock-based compensation expense	1,871						1,871
Net loss	(20,903)								(20,903)
Net change in unrealized loss on investments	(9)									$ (9)
Balance at Dec. 31, 2016	$ 92,562				$ 22		219,718		(127,169)	(9)
Balance, Shares at Dec. 31, 2016					22,139,346
Balance, Convertible Preferred Stock at Dec. 31, 2016			$ 0
Balance, Convertible Preferred Stock, Shares at Dec. 31, 2016			0
Issuance of common stock upon the exercise of options, Shares	827,556
Issuance of common stock upon in connection with employee equity incentive plans, net	$ 6,390				$ 1		6,389
Issuance of common stock upon in connection with employee equity incentive plans, net, shares					1,027,595
Issuance of common stock upon net exercise of warrants, Shares					210,374
Tax withholding upon vesting of restricted stock awards	(46)						(46)
Stock-based compensation expense	10,123						10,123
Net loss	(29,420)								(29,420)
Net change in unrealized loss on investments	(56)									(56)
Balance at Dec. 31, 2017	$ 79,553				$ 23		236,184		(156,589)	(65)
Balance, Shares at Dec. 31, 2017	23,377,315				23,377,315
Balance, Convertible Preferred Stock at Dec. 31, 2017			$ 0
Balance, Convertible Preferred Stock, Shares at Dec. 31, 2017			0
Issuance of common stock upon the exercise of options, Shares	798,424
Accounting Standards Codification606 cumulative effectadjustment upon adoption	$ 1,354								1,354
Issuance of common stock upon in connection with employee equity incentive plans, net	9,319						9,319
Issuance of common stock upon in connection with employee equity incentive plans, net, shares					990,758
Tax withholding upon vesting of restricted stock awards	(3,877)						(3,877)
Stock-based compensation expense	16,329						16,329
Net loss	(48,280)								(48,280)
Net change in unrealized loss on investments	24									24
Balance at Dec. 31, 2018	$ 54,422				$ 23		$ 257,955		$ (203,515)	$ (41)
Balance, Shares at Dec. 31, 2018	24,368,073				24,368,073
Balance, Convertible Preferred Stock at Dec. 31, 2018			$ 0
Balance, Convertible Preferred Stock, Shares at Dec. 31, 2018			0

Consolidated Statements of Cash

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash flows from operating activities
Net loss	$ (48,280)	$ (29,420)	$ (20,903)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	2,269	1,597	568
Stock-based compensation	16,329	10,123	1,871
Amortization of debt discount and issuance costs	210	260	251
Amortization (accretion) on investments	(907)	(352)	6
Loss on disposal of assets	75	9
Provision for bad debt and contractual allowance	15,218	9,403	4,686
Change in fair value of preferred stock warrant liabilities			2,123
Non-cash interest expense		1,550	1,481
Loss on extinguishment of debt	3,029
Repayment of interest paid in kind	(3,141)
Changes in operating assets and liabilities:
Accounts receivable	(22,885)	(12,950)	(8,515)
Inventory	(380)	(293)	(245)
Prepaid expenses and other current assets	(1,568)	(795)	(863)
Other assets	269	(467)	(1,235)
Accounts payable	(192)	634	305
Accrued liabilities	10,776	5,364	3,380
Deferred rent	105	135	(2)
Deferred revenue	5	291	441
Net cash used in operating activities	(29,068)	(14,911)	(16,651)
Cash flows from investing activities
Purchases of property and equipment	(5,180)	(3,562)	(2,763)
Purchases of available-for-sale investments	(93,158)	(129,829)	(65,403)
Sales of available-for-sale investments	5,962
Maturities of available-for-sale investments	126,493	98,707
Net cash provided by (used in) investing activities	34,117	(34,684)	(68,166)
Cash flows from financing activities
Proceeds from issuance of common stock upon exercise of stock options, net of repurchases	9,319	6,390	132
Proceeds from issuance of common stock upon exercise of warrants			206
Tax withholding upon vesting of restricted stock awards	(3,877)	(46)
Payments of deferred offering costs		188	(3,522)
Proceeds from long-term debt, net of debt discount and issuance costs	35,000
Repayments of long-term debt	(31,500)
Payments of issuance costs for long term debt	(121)
Proceeds of issuance of common stock upon initial public offering			114,436
Premiums paid on extinguishment of debt	(2,518)
Net cash provided by financing activities	6,303	6,532	111,252
Net increase (decrease) in cash and cash equivalents	11,352	(43,063)	26,435
Cash and cash equivalents, beginning of year	8,671	51,734	25,299
Cash and cash equivalents, end of year	20,023	8,671	51,734
Supplemental disclosures of cash flow information
Interest paid	6,065	1,550	1,591
Non-cash investing and financing activities
Property and equipment included in accounts payable and accrued liabilities	$ 101	$ 35	423
Deferred offering costs included in accounts payable and accrued liabilities			188
Conversion of preferred stock to common stock			97,553
Conversion of preferred stock warrants to common stock warrants			$ 4,821

Organization and Description of

Organization and Description of Business	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Organization and Description of Business	1. Organization and Description of Business iRhythm Technologies, Inc. (the “Company”) was incorporated in the state of Delaware in September 2006. The Company is a digital healthcare company redefining the way cardiac arrhythmias are clinically diagnosed by combining wearable biosensing technology with cloud-based data analytics and deep-learning capabilities. The Company commenced commercial introduction of its products in the United States in 2009 following clearance by the U.S. Food and Drug Administration. The Company is headquartered in San Francisco, California, which also serves as a clinical center. The Company has additional clinical centers in Lincolnshire, Illinois and Houston, Texas and a manufacturing facility in Cypress, California. In March 2016, the Company formed a wholly-owned subsidiary in the United Kingdom. The Company manages its operations as a single operating segment. Substantially all of the Company’s assets are maintained in the United States. The Company derives substantially all of its revenue from sales to customers in the United States. Reverse Stock Split On October 4, 2016, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of the Company’s issued and outstanding common stock at a 1-for- 5.882698 ratio, which was effected on October 5, 2016. The par value and authorized shares of common stock and convertible preferred stock were not adjusted as a result of the reverse split. All issued and outstanding common stock, options to purchase common stock and per share amounts contained in these consolidated financial statements have been retroactively adjusted to reflect the reverse stock split for all periods presented. Initial Public Offering The Company’s initial public offering of 7,238,235 shares of common stock was effected through a registration statement on Form S-1 (Registration Nos. 333-213773 and 333-214179), which was declared effective on October 19, 2016. The initial public offering closed on October 25, 2016 and resulted in net proceeds of approximately $110.7 million, after deducting underwriting discounts and commissions of $8.6 million and other expenses of $3.7 million. In October 2016, immediately upon the Company’s sale of its common stock in the initial public offering

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles and include the accounts of iRhythm Technologies, Inc. and its wholly-owned subsidiary, iRhythm Technologies Ltd., established in March 2016. All intercompany accounts and transactions have been eliminated. The financial statements of iRhythm Technologies Ltd. use the U.S. dollar as the functional currency Use of Estimates The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. On an ongoing basis, management evaluates its estimates, including those related to revenue recognition, contractual allowances for revenue, allowance for doubtful accounts, the useful lives of property and equipment, the recoverability of long-lived assets including the estimated usage of the printed circuit board assemblies (“PCBAs”), accounting for income taxes, the fair value of the Company’s common stock and stock-based compensation. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that the Company believes are reasonable under the circumstances, including assumptions as to future events. Actual results may differ from those estimates. Fair Value of Financial Instruments The carrying amounts of certain of the Company’s financial instruments, which include cash equivalents, short-term investments, long-term investments, accounts receivable, accounts payable and accrued liabilities, approximate fair value due to their short maturities. Cash Equivalents Cash equivalents consist of short-term, highly liquid investments with original maturities of three months or less from the date of purchase. Investments Short-term investments consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than one year as of the balance sheet date. Long-term investments have maturities greater than one year as of the balance sheet date. All investments are carried at fair value based upon quoted market prices. Unrealized gains and losses on available-for-sale securities are excluded from earnings and are reported as a component of accumulated other comprehensive loss. The cost of available-for-sale securities sold is based on the specific-identification method. Realized gains and losses are included in earnings, and are derived for specific-identification method for determining the costs of investments sold. Amortization of premiums and accretion of discounts are reported as a component of interest income and interest expense, respectively. Accounts Receivable, Allowance for Doubtful Accounts and Contractual Allowance Accounts receivable consists of amounts due to the Company from institutions and third-party government and commercial payors and their related patients, as a result of the Company’s normal business activities. Accounts receivable is reported on the balance sheets net of an estimated allowance for doubtful accounts and contractual allowance. The Company establishes an allowance for doubtful accounts for estimated uncollectible receivables based on its historical experience and recognizes the provision as a component of selling, general and administrative expenses. The Company establishes a contractual allowance when it estimates that consideration to be received will be lower than the contracted rate based on its historical experience and recognizes the provision as a reduction to revenue. The following table presents the changes in the allowance for doubtful accounts (in thousands): Year ended December 31, 2018 2017 Balance, beginning of year $ 3,568 $ 1,792 Add: provision for doubtful accounts 5,826 3,640 Less: write-offs, net of recoveries and other adjustments (4,543 ) (1,864 ) Balance, end of year $ 4,851 $ 3,568 The following table presents the changes in the contractual allowance (in thousands): Year ended December 31, 2018 2017 Balance, beginning of year $ 7,444 $ 2,340 Add: allowance for contractual adjustments 9,392 5,763 Less: contractual adjustments (6,235 ) (659 ) Balance, end of year $ 10,601 $ 7,444 The following table presents the impact of allowance for doubtful accounts and contractual allowance on accounts receivable (in thousands): Year ended December 31, 2018 2017 Gross accounts receivable $ 37,429 $ 23,965 Less: allowance for doubtful accounts (4,851 ) (3,568 ) Less: contractual allowance (10,601 ) (7,444 ) Net accounts receivable $ 21,977 $ 12,953 The Company reviews and updates its estimates for the allowances for doubtful accounts and contractual allowance periodically to reflect its experience regarding historical collections. If management were to make different judgments or utilize different estimates in the allowances for doubtful accounts and contractual allowance, differences in the amount of reported selling, general and administrative expenses and revenue could result, respectively. Concentrations of Risk Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist primarily of cash and cash equivalents, investments and accounts receivable. Cash, cash equivalents, and investments are deposited in financial institutions which, at times, may be in excess of federally insured limits. Cash equivalents are invested in highly rated money market funds. The Company invests in a variety of financial instruments, such as, but not limited to, United States Government securities, corporate notes, commercial paper and, by policy, limits the amount of credit exposure with any one financial institution or commercial issuer. The Company has not experienced any material losses on its deposits of cash and cash equivalents or investments. Concentrations of credit risk with respect to accounts receivable are limited due to the large number of customers comprising the Company’s customer base and their dispersion across many geographies. The Company does not require collateral. The Company records an allowance for doubtful accounts when it becomes probable that a receivable will not be collected. Federal government agencies, including CMS and the military, accounted for approximately 37%, 37% and 40% of the Company’s revenue for the years ended December 31, 2018, 2017 and 2016, respectively. Accounts receivable related to federal government agencies accounted for 18% and 27% at December 31, 2018 and 2017, respectively. Supply Risk While the Company has not experienced manufacturing supply disruptions to date, the Inventory Inventory is stated at the lower of cost or net realizable value, cost being determined on a standard cost basis for material costs and on actual cost basis for labor and overhead, which approximates actual cost on a first in, first out (“FIFO”) basis, and market being determined as the lower of cost or net realizable value. The Company records write-downs of inventory that is obsolete or in excess of anticipated demand. The Company also records market value based write-downs on consideration of product lifecycle stage, technology trends, product development plans and assumptions about future demand and market conditions. Actual demand may differ from forecasted demand, and such differences may have a material effect on recorded inventory values. Inventory write-downs are charged to cost of revenue and establish a new cost basis for the inventory. Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the assets, ranging from three to five years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful lives of the assets. Maintenance and repairs are charged to expense as incurred, and improvements and betterments are capitalized. Internal-Use Software The Company capitalizes costs related to internal-use software during the application development stage. Costs related to planning and post implementation activities are expensed as incurred. Capitalized internal-use software is amortized, and recognized as cost of revenue, on a straight-line basis over the estimated useful life, which is up to five years. The Company evaluates the useful lives of these assets on an annual basis, and tests for impairment whenever events or changes in circumstances occur that could impact the recoverability of these assets. Capitalized internal-use software costs are classified as a component of property and equipment. Goodwill Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible net assets acquired in business combinations. Goodwill is tested for impairment on an annual basis and at any other time if events occur or circumstances indicate that the carrying amount of goodwill may not be recoverable. Such events or circumstances may include significant adverse changes in the general business climate, among other things. The impairment test is performed by determining the enterprise fair value of the Company, which is primarily based on the Company’s market capitalization. If the Company’s carrying value, as a one reporting unit entity, is less than its fair value, then the fair value is allocated to all of its assets and liabilities (including any unrecognized intangible assets) as if the fair value was the purchase price to acquire the Company. The excess of the fair value over the amounts assigned to the Company’s assets and liabilities is the implied fair value of the goodwill. If the carrying amount of goodwill exceeds the implied fair value of that goodwill, an impairment loss is recognized in an amount equal to that excess. The Company performs its annual evaluation of goodwill during the fourth quarter of each fiscal year. The Company did not record any charges related to goodwill impairment in any of the periods presented in these consolidated financial statements. Impairment of Long-Lived Assets The Company annually reviews long-lived assets for impairment or whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured as the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the asset. To date, there have been no such impairments of long-lived assets. Other Assets Included in the other assets are printed circuit board assemblies (“PCBAs”) totaling $2.5 million and $3.3 million as of December 31, 2018 and 2017, respectively. The Company uses a PCBA in each wearable Zio XT monitor which is used numerous times and has a useful life beyond one year. Each time the PCBA is used in a wearable Zio XT monitor, a portion of the cost of the PCBA is recorded as a cost of revenue. The Company has based its estimates of how many times a PCBA can be used on testing in research and development, loss rates, product obsolescence, and the amount of time it takes the device to go through the manufacturing, shipping, customer shelf and patient wear time and upload process. The Company periodically evaluates the use estimate. Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity during the period from non-owner sources. The Company’s unrealized gains and losses on available-for-sale securities represent the only component of other comprehensive income (loss) that are excluded from the reported net loss and that are presented in the consolidated statements of comprehensive income. Revenue Recognition The Company adopted Accounting Standard Codification Topic 606, Revenue from Contracts with Customers Health Care Entities - Revenue Recognition Revenue Recognition The Company’s revenue is generated primarily from the provision of its cardiac rhythm monitoring service, the Zio XT service. The Zio XT is a cardiac rhythm monitoring service that has a patient wear period of up to 14 days and is billable when the monitoring reports are delivered to the healthcare provider, which is also when the service is complete and the Company recognizes revenue. The time from when the patient has the Zio XT device applied to the time the report is posted is generally around 20 days. The Company accounts for contract revenue with a customer when there is a legally enforceable contract between the Company and the customer, the rights of the parties are identified, the contract has commercial substance, and collectability of the contract consideration is probable. The Company's revenue is measured based on consideration specified in the contract with each customer. A unique aspect of healthcare is the involvement of multiple parties to the service transaction. In addition to the patient, often a third-party, for example a commercial or governmental payor or healthcare institution, like a hospital or clinic, will pay the Company for some or all of the service on the patient’s behalf. Separate contractual arrangements exist between the Company and third-party payors that establish amounts the third-party payor will pay on behalf of a patient for covered services rendered and should be considered in determining collectability and the transaction price for services provided to a patient covered by that third-party payor. The Company recognizes revenue on an accrual basis based on estimates of the amount that will ultimately be realized. These estimates require significant judgment by management. In determining the amount to recognize for a delivered report, the Company considers factors such as claim payment history from both payors and patient out-of-pocket costs, payor coverage, whether there is a contract between the payor or healthcare institution and the Company, historical amount received for the service, and any current developments or changes that could impact reimbursement and healthcare institution payments. A summary of the payment arrangements with third-party payors and healthcare institutions is as follows: • Contracted third-party payors – The Company has contracts with negotiated prices for services provided for patients with commercial healthcare insurance carriers • Centers for Medicare and Medicaid Services (“CMS”) – The Company has received independent diagnostic testing facility approval from regional Medicare Administrative Contractors and will receive reimbursement per the relevant Current Procedural Terminology (“CPT”) code rate for the services rendered to the patient covered by CMS. • Non-contracted third-party payors – Non-contracted commercial and government payors often reimburse out-of-network rates provided under the relevant CPT codes on a case-by-case basis. The transaction price is based on factors including an average of the Company’s historical collection experience for its non-contracted services. This rate is reviewed at least quarterly. • Healthcare institutions – Healthcare institutions are typically hospitals or physician practices in which the Company has negotiated amounts for its monitoring services, including certain governmental agencies such as the Veteran’s Administration and Department of Defense. The Company is utilizing the portfolio approach practical expedient under ASC 606 for revenue recognition. The Company accounts for the contracts within each portfolio as a collective group, rather than individual contracts. Based on history with these portfolios and the similar nature and characteristics of the patients within each portfolio, the Company has concluded that the financial statement effects are not materially different than if accounting for revenue on a contract-by-contract basis. For the healthcare institution and CMS portfolios, the Company has historical experience of collecting substantially all of the negotiated contractual rates and determined at contract inception that these customers, and or their related third-party payor that pays the Company on their behalf, have the intention and ability to pay the promised consideration. As such, the Company is not providing an implicit price concession but, rather, has chosen to accept the risk of default, and any subsequent impairment of the related receivable are recorded as bad debt expense. For contracted portfolios, the Company is providing an implicit price concession because, while the Company has a contract with the underlying payor, the Company expects to accept a lower amount of consideration when claims are adjudicated and allowable claims are determined by the commercial payor. The implicit price concession is recorded as variable consideration to the transaction price and recorded as an adjustment to revenue as a contractual allowance. Historical cash collection indicates that it is probable that substantially all of the allowable claim amount will be received, and hence this amount is recorded as revenue. Any subsequent impairment of the related receivable is recorded as bad debt expense. For non-contracted portfolios, the Company is providing an implicit price concession because the Company does not have a contract with the underlying payor, the result of which requires the Company to estimate transaction price based on historical cash collections utilizing the expected value method. Subsequent adjustments to the transaction price are recorded as an adjustment to revenue and not as bad debt expense. Disaggregation of Revenue The Company disaggregates revenue from contracts with customers by payor type. The Company believes these categories aggregate the payor types by nature, amount, timing and uncertainty of its revenue streams. Disaggregated revenue by payor type and major service line for the year ended December 31, 2018 was as follows (in thousands): Year Ended December 31, 2018 Commercial Payors $ 69,363 Centers for Medicare & Medicaid 40,552 Healthcare Institutions 37,378 Total $ 147,293 Contract Liabilities ASC 606 requires an entity to present a revenue contract as a contract liability when the Company has an obligation to transfer goods or services to a customer for which the Company has received consideration from the customer, or an amount of consideration from the customer is due and unconditional (whichever is earlier). Certain of the Company’s customers pay the Company directly for the Zio XT service upon shipment of devices. Such advance payments, or contract liabilities, are recorded as deferred revenue on the Condensed Consolidated Balance Sheets and revenue is recognized when reports are delivered to physicians. During the year ended December 31, 2018, $1.2 million relating to the contract liability balance at the beginning of 2018 was recognized as revenue. Contract Costs Under ASC 340, the incremental costs of obtaining a contract with a customer are recognized as an asset. Incremental costs of obtaining a contract are those costs that an entity incurs to obtain a contract with a customer that it would not have incurred if the contract had not been obtained. The Company’s current commission programs are considered incremental. However, as a practical expedient, ASC 340 permits the Company to immediately expense contract acquisition costs, as the asset that would have resulted from capitalizing these costs will be amortized in one year or less. Cost of Revenue Cost of revenue is expensed as incurred, and includes direct labor, material costs, equipment and infrastructure expenses, amortization of internal-use software, allocated overhead, and shipping and handling. Direct labor includes payroll and personnel-related costs involved in manufacturing, data analysis, and customer service. Material costs include both the disposable costs of the device and amortization of the PCBAs. Each time the PCBA is used in a wearable Zio XT monitor, a portion of the cost of the PCBA is recorded as a cost of revenue. Research and Development The Company’s research and development costs are expensed as incurred. Research and development costs include, but are not limited to, payroll and personnel-related expenses, laboratory supplies, consulting costs and overhead charges. Income Taxes The Company uses the asset and liability method to account for income taxes in accordance with the authoritative guidance for income taxes. Under this method, deferred tax assets and liabilities are determined based on future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases, and tax loss and credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. On December 22, 2017, the 2017 Tax Cuts and Jobs Act (Tax Act) was enacted into law and the new legislation contains several key tax provisions that affected the Company, including a one-time mandatory transition tax on accumulated foreign earnings and a reduction of the corporate income tax rate to 21% effective January 1, 2018, among others. The Company was required to recognize the effect of the tax law changes in the period of enactment, such as determining the transition tax and remeasuring U.S. deferred tax assets and liabilities. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allowed the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. As a result, the Company previously provided a provisional estimate of the effect of the Tax Act in its financial statements. In the fourth quarter of 2018, the Company completed the analysis to determine the effect of the Tax Act and recorded immaterial adjustments as of December 31, 2018. The Company recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount that has a greater than 50% likelihood of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. The Company records interest and penalties related to unrecognized tax benefits in income tax expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits. Stock-based Compensation The Company measures its stock-based awards made to employees based on the estimated fair values of the awards as of the grant date. The fair value of stock options is determined using the Black-Scholes option pricing model. Stock-based compensation expense is recognized over the requisite service period using the straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. For restricted stock, the compensation cost for these awards is based on the closing price of the Company’s common stock on the date of grant, and recognized as compensation expense on a straight-line basis over the requisite service period. The Company recognizes compensation expense related to the Employee Stock Purchase Program (“ESPP”) based on the estimated fair value of the options on the date of grant, net of estimated forfeitures. The Company estimates the grant date fair value, and the resulting stock-based compensation expense, using the Black-Scholes option pricing model for each purchase period. The grant date fair value is expensed on a straight-line basis over the offering period. Net Loss per Common Share Basic net loss per common share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per common share is the same as basic net loss per common share for all periods presented, since the effect of potentially dilutive securities are anti-dilutive. Recently Adopted Accounting Guidance In May 2014, the Financial Accounting Standards Board (“FASB”), issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) The Company adopted this standard on January 1, 2018 and used the modified retrospective approach. Upon adoption, the Company recognized the cumulative effect of $1.4 million as an adjustment to decrease the opening balance of the Company’s accumulated deficit. This adjustment did not have a material impact on the Company’s consolidated financial statements. Prior periods were not retrospectively adjusted. The following table presents the impact of adoption of ASU 2014-09 on the Consolidated Statement of Operations and the Consolidated Balance Sheet (in thousands): Year Ended December 31, 2018 Consolidated Statement of Operations Impact: As Reported Without the adoption of Topic 606 Impact Revenue $ 147,293 $ 145,320 $ 1,973 Sales, General & Administrative Expense $ 131,582 $ 132,603 $ (1,021 ) Net Loss $ (48,280 ) $ (51,274 ) $ 2,994 December 31, 2018 Consolidated Balance Sheet Impact: As Reported Without the adoption of Topic 606 Impact Accounts Receivable, net $ 21,977 $ 17,629 $ 4,348 Accumulated Deficit $ (203,515 ) $ (207,863 ) $ 4,348 In November 2016, the FASB issued ASU No. 2016-18 Statement of Cash Flow: Restricted Cash (Topic 230), which provides guidance on the classification of restricted cash to be included with cash and cash equivalents when reconciling the beginning of period and end of period total amounts on the statement of cash flows. The amendments of this ASU are effective for interim and annual periods beginning after December 15, 2017. The standard must be applied retrospectively to all periods presented and the Company adopted the standard on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s Condensed Consolidated Statement of Cash Flows. Recent Accounting Pronouncements Not Yet Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (“Topic 842”), which requires lessees to recognize lease liabilities and corresponding right-of-use assets on the Consolidated Balance Sheet for all leases. For finance leases, the lessee would recognize interest expense and amortization of the right-of-use asset and, for operating leases, the lessee would recognize a straight-line lease expense. It also changes the definition of a lease and expands the disclosure requirements of lease arrangements. The Company is currently evaluating the impact of the adoption of this standard, and expects that the impact on the Consolidated Balance Sheet will be material as a result of recognizing right of use assets and liabilities for existing real estate leases. The Company has no embedded leases with suppliers. In addition, the standard will require changes to the Company’s current accounting policies, processes, and internal controls. The Company expects to adopt the following practical expedients allowed under Topic 842: • The package of three practical expedients, which allows entities to make an election that allows them not to reassess (1) embedded leases not classified as leases under Topic 842, (2) Lease classification of existing or expiring leases, and (3) indirect costs for existing or expired leases. • Combining lease and non-lease components practical expedient, which allows lessees, as an accounting policy election by class of underlying asset, to choose not to separate non-lease components from lease components and instead to account for each separate lease component and the non-lease components associated with that lease component as a single lease component. • Comparative reporting practical expedient, which allows entities to initially apply Topic 842 at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company will adopt Topic 842 in the first quarter of fiscal

Cash Equivalents and Investment

Cash Equivalents and Investments	12 Months Ended
	Dec. 31, 2018
Cash Equivalents And Investments [Abstract]
Cash Equivalents and Investments	3. Cash Equivalents and Investments The fair value of securities, not including cash at December 31, 2018 and 2017 were as follows (in thousands): December 31, 2018 Amortized Gross Unrealized Estimated Cost Gains Losses Fair Value Money market funds $ 10,606 $ — $ — $ 10,606 U.S. government securities 9,976 — (1 ) 9,975 Corporate notes 16,514 3 (18 ) 16,499 Commercial paper 36,331 — — 36,331 Total available-for-sale marketable debt securities $ 73,427 $ 3 $ (19 ) $ 73,411 Classified as: Cash equivalents $ 15,091 Short-term investments 58,320 Total $ 73,411 December 31, 2017 Amortized Gross Unrealized Estimated Cost Gains Losses Fair Value Money market funds $ 5,574 $ — $ — $ 5,574 U.S. government securities 24,969 — (29 ) 24,940 Corporate notes 24,539 — (36 ) 24,503 Commercial paper 47,243 — — 47,243 Total available-for-sale marketable debt securities $ 102,325 $ — $ (65 ) $ 102,260 Classified as: Cash equivalents $ 5,574 Short-term investments 93,692 Long-term investments 2,994 Total $ 102,260 Available-for-sale securities held as of December 31, 2018 had a weighted average days to maturity of 62 days. As of December 31, 2018, the Company had 8 investments in an unrealized loss position. There have been no material realized gains or realized losses on available-for-sale securities for the periods presented. As the carrying value approximates the fair value for the Company’s cash equivalents, short-term and long-term marketable securities shown in the tables above, the following table summarizes the fair value of the Company’s cash equivalents, short-term and long-term marketable securities classified by maturity as of December 31, 2018 and 2017 (in thousands): December 31, 2018 2017 Due within one year $ 73,411 $ 99,266 Due after one year through three years — 2,994 Total available-for-sale marketable debt securities $ 73,411 $ 102,260 The following tables present the Company's available-for-sale securities that were in an unrealized loss position as of December 31, 2018 and December 31, 2017 (in thousands): December 31, 2018 Less than 12 months 12 Months or Greater Assets Fair Value Unrealized Loss Fair Value Unrealized Loss U.S. government securities $ 5,977 $ (1 ) $ — $ — Corporate notes 11,521 (10 ) 2,993 (8 ) Total $ 17,498 $ (11 ) $ 2,993 $ (8 ) December 31, 2017 Less than 12 months Assets Fair Value Unrealized Loss U.S. government securities $ 24,940 $ (29 ) Corporate notes 24,503 (36 ) Commercial paper 7,866 — Total $ 57,309 $ (65 )

Fair Value Measurements

Fair Value Measurements	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Fair Value Measurements	4. Fair Value Measurements The Company discloses and recognizes the fair value of its assets and liabilities using a hierarchy that prioritizes the inputs to valuation techniques used to measure fair value. Fair value is defined as the price that would be received to sell an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the reporting date. The hierarchy gives the highest priority to valuations based upon unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurements) and the lowest priority to valuations based upon unobservable inputs that are significant to the valuation (Level 3 measurements). The guidance establishes three levels of the fair value hierarchy as follows: Level 1 —Inputs are unadjusted quoted prices in active markets for identical assets or liabilities at the measurement date. Level 2 —Inputs (other than quoted market prices included in Level 1) are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument’s anticipated life. Level 3 —Inputs reflect management’s best estimate of what market participants would use in pricing the asset or liability at the measurement date. Consideration is given to the risk inherent in the valuation technique and the risk inherent in the inputs to the model. Assets and liabilities measured at fair value are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement in its entirety requires management to make judgments and consider factors specific to the asset or liability. The corporate notes, commercial paper and government bonds are classified as Level 2 as they were valued based upon quoted market prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active and model-based valuation techniques for which all significant inputs are observable in the market or can be corroborated by observable market data for substantially the full term of the assets. The fair value of the Company’s outstanding interest-bearing obligations is estimated using the net present value of the payments, discounted at an interest rate that is consistent with market interest rates, which is a Level 2 input. Both the carrying amount and the estimated fair value of the Company’s outstanding interest-bearing obligations at December 31, 2018 were $34.9 million and $34.9 million. The carrying amount and the estimated fair value of the Company’s outstanding interest-bearing obligations at December 31, 2017 were $34.0 million and $38.6 million, respectively. The Company had no transfers between levels of the fair value hierarchy of its assets measured at fair value. The following table presents the fair value of the Company’s financial assets determined using the inputs defined above (in thousands). December 31, 2018 Level 1 Level 2 Level 3 Total Assets Money market funds $ 10,606 $ — $ — $ 10,606 U.S. government securities — 9,975 — 9,975 Corporate notes — 16,499 — 16,499 Commercial paper — 36,331 — 36,331 Total $ 10,606 $ 62,805 $ — $ 73,411 December 31, 2017 Level 1 Level 2 Level 3 Total Assets Money market funds $ 5,574 $ — $ — $ 5,574 U.S. government securities — 24,940 — 24,940 Corporate notes — 24,503 — 24,503 Commercial paper — 47,243 — 47,243 Total $ 5,574 $ 96,686 $ — $ 102,260

Balance Sheet Components

Balance Sheet Components	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Balance Sheet Components	5. Balance Sheet Components Inventory Inventory and Printed Circuit Board Assemblies (“PCBAs”), which are recorded as other assets as they are used many times over a period longer than one year on average, consisted of the following (in thousands): December 31, 2018 2017 Raw materials $ 1,028 $ 1,103 Finished goods 3,565 3,830 Total $ 4,593 $ 4,933 December 31, 2018 2017 Reported on the consolidated balance sheet as: Inventory $ 2,062 $ 1,683 Other assets 2,531 3,250 Total $ 4,593 $ 4,933 Amounts reported as other assets are comprised of the PCBA costs that are included in both raw materials and finished goods totals above. Property and Equipment, Net Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Laboratory and manufacturing equipment $ 2,750 $ 1,994 Computer equipment and software 1,062 1,015 Furniture and fixtures 925 953 Leasehold improvements 726 726 Internal-use software 8,925 4,598 Total property and equipment, gross 14,388 9,286 Less: accumulated depreciation and amortization (5,230 ) (3,065 ) Total property and equipment, net $ 9,158 $ 6,221 Depreciation and amortization expense for the years ended December 31, 2018, 2017 and 2016 was $2.3 million, $1.6 million, and $0.6 million, respectively. Accrued Liabilities Accrued liabilities consisted of the following (in thousands): December 31, 2018 2017 Accrued vacation $ 2,825 $ 2,081 Accrued payroll and related expenses 18,188 9,361 Accrued professional services fees 1,243 1,041 Claims payable 2,374 1,375 Other 1,940 1,786 Total accrued liabilities $ 26,570 $ 15,644

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Commitments and Contingencies	6. Commitments and Contingencies Lease Arrangements The Company leases office and manufacturing space under non-cancelable operating leases which expire on various dates through 2031. These leases generally contain scheduled rent increases or escalation clauses and renewal options. The Company recognizes rent expense on a straight-line basis over the lease period. The following table summarizes the Company’s future minimum lease payments as of December 31, 2018 (in thousands): Year Ending December 31: 2019 $ 8,135 2020 10,669 2021 10,828 2022 11,150 2023 11,483 Thereafter 98,209 Total $ 150,474 On October 4, 2018, the Company entered into an office lease (“San Francisco Lease”) to rent approximately 117,560 rentable square feet in San Francisco, California, which will become the Company’s new headquarters. The Company’s current headquarters is in the same building as the San Francisco Lease. The San Francisco lease is expected to commence on or around April 1, 2019 and has a twelve-year term, which will expire on August 31, 2031. The Company is entitled to one option to extend the San Francisco Lease for a five-year term, subject to certain requirements. In addition, the landlord will provide a tenant improvement allowance of up to $2.4 million for leasehold improvements in connection with the cost of construction of the initial alterations within the premises. Annual rental payments will be $10.0 million, with a 3% increase each year and will be capitalized in accordance with Topic 842. Annual rental payments on the San Francisco Lease have been included in the table above. The Company has obtained a standby letter of credit The Company’s rent expense was $5.5 million, $5.1 million and $3.1 million for the years ended December 31, 2018, 2017 and 2016, respectively. Legal Proceedings From time to time, the Company may become involved in legal proceedings arising from the ordinary course of its business. Management is currently not aware of any matters that could have a material adverse effect on the financial position, results of operations or cash flows of the Company. Indemnifications In the ordinary course of business, the Company enters into agreements that may include indemnification provisions. Pursuant to such agreements, the Company may indemnify, hold harmless and defend an indemnified party for losses suffered or incurred by the indemnified party. Some of the provisions will limit losses to those arising from third-party actions. In some cases, the indemnification will continue after the termination of the agreement. The maximum potential amount of future payments the Company could be required to make under these provisions is not determinable. The Company has also entered into indemnification agreements with its directors and officers that may require the Company to indemnify its directors and officers against liabilities that may arise by reason of their status or service as directors or officers to the fullest extent permitted by California corporate law. The Company currently has directors’ and officers’ insurance. The Company has never incurred material costs to defend lawsuits or settle claims related to these indemnification provisions, and believes that the estimated fair value of these indemnification obligations is not material and it has not accrued any amounts for these obligations.

Debt

Debt	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Debt	7. Debt Pharmakon Loan Agreement In December 2015, the Company entered into a Loan Agreement with Biopharma Secured Investments III Holdings Cayman LP (the “Pharmakon Loan Agreement”). The Pharmakon Loan Agreement provides for up to $55.0 million in term loans split into two tranches as follows: (i) the Tranche A Loans are $30.0 million in term loans, and (ii) the Tranche B Loans are up to $25.0 million in term loans. The Tranche A Loans were drawn on December 4, 2015. The Tranche B Loans were available to be drawn prior to December 4, 2016. No additional draw was taken. The Tranche A Loans bear interest at a fixed rate equal to 9.50% per annum that is due and payable quarterly in arrears. During the first eight calendar quarters, 50% of the interest due and payable was added to the then outstanding principal. In December 2015, the Company used the proceeds from the Pharmakon Loan Agreement to repay $4.9 million of bank debt to SVB. The issuance costs and debt discount have been netted against the borrowed funds on the balance sheet. On October 23, 2018, the Company repaid the principal amount of the Tranche A Loan of $30.0 million and related accrued interest of $3.3 million. The Company incurred a $3.0 million loss in connection with the early extinguishment of the Pharmakon Loan Agreement which included a prepayment premium fee of $1.0 million and additional consideration related to the prepayment of $1.5 million. The debt balance as of December 31, 2017 was $32.5 million. Bank Debt In December 2015, the Company entered into a Second Amended and Restated Loan and Security Agreement with SVB, (the “SVB Loan Agreement”). Under the SVB Loan Agreement the Company may borrow, repay and reborrow under a revolving credit line, but not in excess of the maximum loan amount of $15.0 million, until December 4, 2018, when all outstanding principal and accrued interest becomes due and payable. Any principal amount outstanding under the SVB Loan Agreement shall bear interest at a floating rate per annum equal to the rate published by The Wall Street Journal as the “Prime Rate” plus 0.25%. The Company may borrow up to 80% of its eligible accounts receivable, up to the maximum of $15.0 million. In August 2016, the Company obtained a $3.1 million standby letter of credit pursuant to the SVB Loan Agreement in connection with a lease for its San Francisco office. In October 2018, the Company entered into the Third Amended and Restated Loan and Security Agreement with Silicon Valley Bank (“Third Amended and Restated SVB Loan Agreement”). This Agreement amends and restates the Second Amended and Restated Loan and Security Agreement between the Company and Silicon Valley Bank (“SVB”) dated December 4, 2015, as amended by the First Loan Modification Agreement between the Company and SVB dated August 22, 2016. Pursuant to the Third Amended and Restated SVB Loan Agreement, the Company obtained a term loan (“SVB Term Loan”) for $35.0 million. Total proceeds from the SVB Term Loan were used to pay off the loan agreement with Biopharma Secured Investments III Holdings Cayman LP (“Pharmakon”), totaling $35.8 million. The Company will make interest-only payments through October 31, 2020, followed by 36 monthly payments of principal plus interest on the SVB Term Loan. Interest charged on the SVB Term Loan will be the greater of (a) a floating rate based on the “Prime Rate” published by The Wall Street Journal minus 0.75%, or (b) 4.25%. Under the Third Amended and Restated SVB Loan Agreement, the Company may borrow, repay, and reborrow under a revolving credit line, but not in excess of the maximum loan amount of $25.0 million, which includes an $11.0 million standby letter of credit sublimit availability. In October 2018, a $6.9 million standby letter of credit was obtained in connection with a lease for the Company’s San Francisco headquarters. Any principal amount outstanding under the Third Amended and Restated SVB Loan Agreement revolving credit line shall bear interest at an amount that is the greater of (a) a floating rate per annum equal to the rate published by The Wall Street Journal as the “Prime Rate” or (b) 5.00%. The Company may borrow up to 75% of eligible accounts receivable, up to the maximum of $25.0 million. As of December 31, 2018, the Company was eligible to borrow up to $1.7 million and no amount was outstanding under the revolving credit line. The Third Amended and Restated Loan Agreement requires the Company to maintain a minimum consolidated liquidity ratio or minimum adjusted Earnings Before Interest, Tax, Depreciation, and Amortization during the term of the loan facility. In addition, the SVB Loan Agreement contains customary affirmative and negative covenants and events of default. The Company was in compliance with loan covenants as of December 31, 2018. California HealthCare Foundation Note In November 2012, the Company entered into a Note Purchase Agreement and Promissory Note with the California HealthCare Foundation (the “CHCF Note”), through which the Company borrowed $1.5 million. The CHCF Note accrued simple interest of 2.0%. The accrued interest and the principal was to mature in November 2016. In partial consideration for the issuance of the CHCF Note, the Company issued warrants to purchase 22,807 shares of the Company’s Series D convertible preferred stock. In June 2015, the Company amended the CHCF Note to extend the maturity date to May 2018. The CHCF Note was subordinate to other debt. The debt balance, net of debt discount, as of December 31 2017 was $1.5 million. In May 2018, the Company repaid the principal amount of $1.5 million and related $0.2 million in accrued interest on the CHCF Note. Future minimum payments Future minimum payments under the Third Amended and Restated Loan and Security Agreement with Silicon Valley Bank at December 31, 2018 are as follows (in thousands): Year Ending December 31: 2019 $ 1,662 2020 3,603 2021 12,983 2022 12,429 2023 9,934 Total 40,611 Less: Amount representing interest (5,611 ) Less: Amount representing debt issuance costs (101 ) Total carrying value $ 34,899 The total carrying value of debt is classified as long-term on the Consolidated Balance Sheet as of December 31, 2018.

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Income Taxes	8. Income Taxes On December 22, 2017, the 2017 Tax Cuts and Jobs Act (“Tax Act”) was enacted into law and the new legislation contains several key tax provisions that affected the Company, including a one-time mandatory transition tax on accumulated foreign earnings and a reduction of the corporate income tax rate to 21% effective January 1, 2018, among others. The Company was required to recognize the effect of the tax law changes in the period of enactment, such as determining the transition tax and remeasuring its U.S. deferred tax assets and liabilities. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allowed the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. As a result, the Company previously provided a provisional estimate of the effect of the Tax Act in its financial statements. In the fourth quarter of 2018, the Company completed the analysis to determine the effect of the Tax Act and recorded immaterial adjustments as of December 31, 2018. There was no provision for the years ended December 31, 2017 and 2016. The following table presents components of the Company’s provision for income taxes as of December 31, 2018 (in thousands): Year Ended December 31, 2018 Current: Federal $ — State — Foreign 80 Total current tax expense 80 Deferred: Federal — State — Foreign (36 ) Total deferred tax benefit (36 ) Provision for income taxes $ 44 The following table presents a reconciliation of the tax expense computed at the statutory federal rate and the Company’s tax expense for the period presented (in thousands): Year Ended December 31, 2018 2017 2016 Tax at statutory federal rate (10,143 ) (10,034 ) $ (7,107 ) Stock-based compensation (8,562 ) (7,634 ) 255 Meals and Entertainment 309 198 134 Other 149 (145 ) 782 Tax credits (1,015 ) (644 ) (139 ) 2017 Tax Act 44 21,928 — Change in valuation allowance 19,262 (3,669 ) 6,075 Provision for income taxes $ 44 $ — $ — ________________ (1) Deferred income taxes reflect the net effects of temporary differences between the carrying amounts of assets and liabilities for financial reporting purposes and the amounts used for income tax purposes. Significant components of the Company’s deferred tax assets and liabilities are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 50,334 $ 35,338 Tax credit carryforwards 4,287 2,642 Allowances and other 11,694 6,682 Depreciation and amortization (388 ) (414 ) Total deferred tax assets 65,927 44,248 Valuation allowance (65,886 ) (44,248 ) Net deferred tax assets $ 41 $ — Due to the uncertainties surrounding the realization of deferred tax assets through future taxable income, the Company has provided a full valuation allowance against its U.S. deferred tax assets, and, therefore, no benefit has been recognized for the net operating loss carryforwards and other deferred tax assets. The U.S. valuation allowance increased by $21.7 million and decreased by $0.6 million for the years ended December 31, 2018 and December 31, 2017, respectively. The current year change in the U.S. valuation allowance is primarily related to the increase in net operating loss carryforwards generated during the year. The Company recorded an immaterial deferred tax asset related to the Company’s foreign operations in the United Kingdom. The valuation allowance for deferred tax assets consisted of the following activity for the years ended December 31, 2018, 2017 and 2016 (in thousands): Balance at beginning of year Additions Deductions Balance at end of year Year ended December 31, 2016 $ 38,565 $ 6,296 $ — $ 44,861 Year ended December 31, 2017 44,861 — 613 44,248 Year ended December 31, 2018 $ 44,248 $ 21,638 $ — $ 65,886 As of December 31, 2018, the Company had approximately $206.7 million of federal and $107.4 million of state net operating loss carryforwards available to offset future taxable income which expires in varying amounts beginning in 2027 and 2019, respectively. As of December 31, 2018, the Company had tax credit carryforwards of approximately $3.7 million, and $2.4 million available to reduce future taxable income, if any, for both federal and state purposes, respectively. The federal tax credit carryforwards expire beginning in 2028 and the state tax credits can be carried forward indefinitely. Section 382 of the Internal Revenue Code, and similar state provisions, limits the use of net operating loss and tax credit carryforwards in certain situations where equity transactions result in a change of ownership as defined by Internal Revenue Code Section 382. In the event the Company should experience an ownership change, as defined, utilization of its net operating loss carryforwards and tax credits could be limited. A reconciliation of the Company’s unrecognized tax benefit amount is as follows (in thousands): Year Ended December 31, 2018 2017 2016 Balance at beginning of year $ 943 $ 616 $ 570 Additions for tax positions taken in current year 441 328 75 Increases in balance related to prior year tax positions 75 — — Decreases in balances related to prior year tax position — (1 ) (29 ) Balance at end of year $ 1,459 $ 943 $ 616 The total amount of gross unrecognized tax benefits was $1.5 million, $0.9 million, and $0.6 million as of December 31 2018, 2017, and 2016 respectively, all of which , if recognized, would affect the effective tax rate. The Company does not anticipate the total amounts of unrecognized tax benefits will significantly increase or decrease in the next 12 months. The Company’s policy is to include interest and penalties related to unrecognized tax benefits within the provision for taxes. Management determined that no accrual for interest or penalties was required as of December 31, 2018 and 2017. The Company files income tax returns in the U.S. and UK jurisdictions. All of the Company's tax years are open to examination by the US federal, state and foreign tax authorities.

Stockholder's Equity

Stockholder's Equity	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Stockholders' Equity	9. Stockholders’ Equity Common stock The Company’s amended and restated certificate of incorporation dated October 25, 2016, authorizes the Company to issue 100,000,000 shares of common stock with a par value of $0.001 per share and 5,000,000 shares of preferred stock with a par value of $0.001 per share. The holders of common stock are entitled to receive dividends whenever funds and assets are legally available and when declared by the board of directors, subject to the prior rights of holders of all series of convertible preferred stock outstanding. No dividends were declared through December 31, 2018. The Company had reserved shares of common stock for issuance as follows: December 31, 2018 2017 Options issued and outstanding 2,094,137 2,601,181 Unvested restricted stock units 547,891 468,426 Common stock warrants issued and outstanding 4,857 4,857 Shares available for grant under future stock plans 5,607,014 4,650,669 Total 8,253,899 7,725,133

Stock Incentive Plans

Stock Incentive Plans	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock Incentive Plans	10. Stock Incentive Plans 2006 Plan In October 2006, the Company adopted the 2006 Equity Incentive Plan, as amended, (the “2006 Plan”). The Plan provided for the granting of stock options to employees and non-employees of the Company. Options granted under the Plan were either incentive stock options or nonqualified stock options. Incentive stock options (“ISO”) were granted only to employees (including officers and directors who are also employees). Nonqualified stock options (“NSO”) may be granted to employees and non-employees. The board of directors had the authority to determine to whom options will be granted, the number of options, the term and the exercise price. Options under the Plan were granted for periods of up to ten years and at the fair value of the shares on the date of grant as determined by the board of directors. In general, options become exercisable at a rate of 25% after the first anniversary of the grant and then monthly vesting for an additional three years from date of grant. The term for options is no longer than five years for ISOs for which the grantee owns greater than 10% of the voting power of all classes of stock and no longer than ten years for all other options. The Company issues new shares upon the exercise of options. 2016 Plan In October 2016, the Company adopted the 2016 Equity Incentive Plan, (the “2016 Plan”). The 2016 Plan was subsequently approved by the Company’s stockholders and became effective on October 19, 2016, immediately before the effective date of the IPO. Following the effectiveness of the 2016 Plan, no additional options will be granted under the 2006 Plan. In addition, to the extent that any awards outstanding or subject to vesting restrictions under the 2006 Plan are subsequently forfeited or terminated for any reason before being exercised or settled, the shares of common stock reserved for issuance pursuant to such awards as of the closing of the IPO will become available for issuance under the 2016 Plan. The remaining shares available for grant under the 2006 Plan became available for issuance under the 2016 Plan upon the closing of the IPO. On the first day of each year, the 2016 Plan authorizes an annual increase of the least of 3,865,000 shares, 5% of outstanding shares on the last day of the immediately preceding fiscal year or an amount as determined by the Company's Board of Directors. As of December 31, 2018, the Company has reserved 6,140,831 shares of common stock for issuance under the 2016 Plan. Pursuant to the 2016 Plan, stock options, restricted shares, stock units, including restricted stock units and stock appreciation rights may be granted to employees, consultants, and outside directors of the Company. Options granted may be either ISOs or NSOs. Stock options are governed by stock option agreements between the Company and recipients of stock options. ISOs and NSOs may be granted under the 2016 Plan at an exercise price of not less than 100% of the fair market value of the common stock on the date of grant, determined by the Compensation Committee of the Board of Directors. Options become exercisable and expire as determined by the Compensation Committee, provided that the term of ISOs may not exceed ten years from the date of grant. Employee Stock Purchase Program (“ESPP”) In October 2016, the Company’s Board of Directors and stockholders approved the Employee Stock Purchase Plan (the “ESPP”). Under the ESPP, the Company initially reserved 483,031 shares of common stock for issuance as of its effective date of October 19, 2016. On the first day of each calendar year, the number of shares reserved increases by the least of 966,062 shares, 1.5% of the shares of the Company’s common stock outstanding on the last day of the immediately preceding fiscal year, or an amount as determined by the Company’s Board of Directors. The ESPP allows eligible employees to purchase shares of the Company’s common stock at a discount through payroll deductions of up to 15% of their eligible compensation, subject to any plan limitations. The ESPP provides for twelve-month offering periods that each contain two six-month purchase periods. At the end of each purchase period, employees are able to purchase shares at 85% of the lower of the fair market value of the Company’s common stock on the first trading day of the offering period or on the last day of the purchase period. As of December 31, 2018, 276,058 shares of common stock have been issued to employees participating in the ESPP and 889,722 shares were available for issuance under the ESPP. The Company used the following assumptions to estimate the fair value of the ESPP offered for the year ended December 31, 2018: expected term of 0.5 – 1.0 years, volatility of 41.46% - 44.55%, risk-free interest rate of 2.19% - 2.64% and expected dividend yield of zero. Equity Incentive Plan Activity A summary of share-based awards available for grant under the 2016 Plan is as follows: Shares for Grant Balance at December 31, 2015 331,938 Additional options authorized 3,865,000 Awards granted (466,914 ) Awards forfeited 13,013 Balance at December 31, 2016 3,743,037 Additional awards authorized 1,106,966 Awards granted (837,436 ) Awards forfeited 21,585 Balance at December 31, 2017 4,034,152 Additional awards authorized 1,168,865 Awards granted (666,913 ) Awards forfeited 124,478 Awards withheld for tax purposes 56,710 Balance at December 31, 2018 4,717,292 The following table summarizes stock option activity under the 2006 and 2016 Plans, including grants to nonemployees: Options Outstanding Options Outstanding Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Life (years) Aggregate Intrinsic Value (in Balance at December 31, 2015 2,685,913 $ 4.81 7.63 $ 11,589 Options granted 361,385 $ 15.65 Options exercised (57,067 ) $ 2.33 Options forfeited (13,013 ) $ 6.92 Balances at December 31, 2016 2,977,218 $ 6.16 6.93 $ 70,979 Options granted 465,271 $ 36.76 Options exercised (827,556 ) $ 4.11 Options forfeited (13,752 ) $ 14.43 Balance at December 31, 2017 2,601,181 $ 12.24 7.17 113,958 Options granted 366,928 $ 68.32 Options exercised (798,424 ) $ 7.19 Options forfeited (75,548 ) $ 34.30 Balance at December 31, 2018 2,094,137 $ 23.20 7.02 97,976 Options exercisable – December 31, 2018 1,278,367 $ 10.27 6.18 75,695 Options vested and expected to vest – December 31, 2018 2,050,504 $ 22.60 6.99 97,085 The aggregate intrinsic values of options outstanding, exercisable, vested and expected to vest were calculated as the difference between the exercise price of the options and the closing price of the Company’s common stock. During the years ended December 31, 2018, 2017 and 2016, the Company granted options with a weighted-average grant date fair value of $32.38, $18.69 and $8.76 per share, respectively. The aggregate intrinsic value of options exercised was $52.4 million, $32.5 million and $0.7 million for the years ended December 31, 2018, 2017 and 2016, respectively. The total estimated grant date fair value of options vested during the period was $5.3 million, $2.4 million and $1.8 million for the years ended December 31, 2018, 2017 and 2016, respectively. The fair value of non-vested restricted stock units (“RSUs”) is based on the Company’s closing stock price on the date of grant. A summary for the year ended December 31, 2018 Shares Underlying RSUs Weighted Average Grant Date Fair Value Weighted Remaining Vesting Period (in years) Aggregate Intrinsic Value (in Non-vested as of December 31, 2017 468,426 $ 36.96 2.54 $ 26,255 Granted 299,985 $ 71.54 Vested and released (114,880 ) $ 31.53 Forfeited (105,640 ) $ 38.81 Non-vested as of December 31, 2018 547,891 $ 56.62 2.45 $ 38,067

Stock-Based Compensation

Stock-Based Compensation	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Stock-Based Compensation	11. Stock-Based Compensation Employee Stock-Based Compensation The Company estimates the fair value of stock options using the Black-Scholes option valuation model. The fair value of employee stock options is being amortized on a straight-line basis over the requisite service period of the awards. The fair value of employee stock options was estimated using the weighted average assumptions below. Each of these inputs is subjective and its determination generally requires significant judgment. Year Ended December 31 2018 2017 2016 Expected term (in years) 6.1 6.1 6.1 Expected volatility 45.7 % 51.9 % 60.0 % Risk-free interest rate 2.75 % 2.07 % 1.42 % Dividend yield 0.0 % 0.0 % 0.0 % Fair Value of Common Stock — Prior to the completion of the Company’s IPO, the fair value of the shares of the Company’s common stock underlying the stock options had historically been determined by the Company’s board of directors. Because there had been no public market for the Company’s common stock, its board of directors determined the fair value of the Company’s common stock at the time of grant of the option by considering a number of objective and subjective factors, including valuations of comparable companies, sales of the Company’s convertible preferred stock, the Company’s operating and financial performance, the lack of liquidity of the Company’s capital stock, and the general and industry-specific economic outlooks. For stock options granted after the completion of the IPO, the Company’s Board of Directors determined the fair value of each share of underlying common stock based on the closing price of the Company’s common stock as reported on the date of grant. Expected Term —The expected term represents the period that the share-based awards are expected to be outstanding. As the Company has very limited historical information to develop reasonable expectations about future exercise patterns and post-vesting employment termination behavior for its stock-option grants the Company has elected to use the “simplified method” as prescribed by authoritative guidance to compute expected term. Expected Volatility —Since the Company does not have sufficient trading history for its common stock, the expected volatility was estimated based on the average volatility for comparable publicly traded companies over a period equal to the expected term of the stock option grants. When selecting comparable publicly traded companies in a similar industry on which it has based its expected stock price volatility, the Company selected companies with comparable characteristics to it, including enterprise value, risk profiles, position within the industry, and with historical share price information sufficient to meet the expected life of the stock-based awards. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. Risk-Free Interest Rate —The risk-free interest rate is based on the U.S. Treasury yield curve in effect on the date of grant for zero coupon U.S. Treasury notes with maturities approximately equal to expected term of the option award. Expected Dividend Yield —The Company has never paid dividends on its common stock and has no plans to pay dividends on its common stock. Therefore, the Company used an expected dividend yield of zero In addition to the assumptions used in the Black-Scholes option-pricing model, the Company also estimates a forfeiture rate to calculate the stock-based compensation for the Company’s equity awards. The Company will continue to use judgment in evaluating the expected volatility, expected terms and forfeiture rates utilized for the Company’s stock-based compensation calculations on a prospective basis. The following table summarizes the total stock-based compensation expense for options, RSUs and ESPP included in the consolidated statements of operations for all periods presented (in thousands): Year Ended December 31 2018 2017 2016 Cost of revenue $ 193 $ 163 $ 17 Research and development 3,057 1,733 203 Selling, general and administrative 13,079 8,227 1,651 Total stock-based compensation expense $ 16,329 $ 10,123 $ 1,871 As of December 31, 2018, there was total unamortized compensation costs of $14.1 million, net of estimated forfeitures, related to unvested stock options, which the Company expects to recognize over a period of approximately 2.3 years, $21.8 million, net of estimated forfeitures, related to unrecognized RSU expense, which the Company expects to recognize over a period of 2.5 years, and $1.3 million unrecognized ESPP expense, which the Company will recognize over 0.9 years. Non-Employee Stock-Based Compensation Stock based compensation expense related to stock options granted to nonemployees is recognized as the stock options are earned. The measurement of stock based compensation for non-employees is subject to periodic adjustment as the underlying equity instruments vest, and the related compensation expense is based on the estimated fair value of the equity instruments using the Black Scholes option pricing model. The Company believes that the estimated fair value of the stock options is more readily measurable than the fair value of the services received. Such expense was not material for the years ended December 31, 2018, 2017 and 2016.

Net Loss Per Common Share

Net Loss Per Common Share	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Net Loss Per Common Share	12. Net Loss Per Common Share As the Company had net losses for the years ended December 31, 2018, 2017 and 2016, all potential common shares were determined to be anti-dilutive. The following table sets forth the computation of the basic and diluted net loss per share during the years ended December 31, 2018, 2017 and 2016 (in thousands, except share and per share data): Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (48,280 ) $ (29,420 ) $ (20,903 ) Denominator: Weighted-average shares used to compute net loss per common share, basic and diluted 23,885,858 22,627,327 5,285,847 Net loss per common share, basic and diluted $ (2.02 ) $ (1.30 ) $ (3.95 ) The following outstanding shares of potentially dilutive securities have been excluded from diluted net loss per common share for the years ended December 31, 2018, 2017 and 2016 because their inclusion would be anti-dilutive: Year Ended December 31, 2018 2017 2016 Options to purchase common stock 2,094,137 2,601,181 2,977,218 RSUs issued and unvested 547,891 468,426 105,529 Warrants to purchase common stock 4,857 4,857 217,245 Total 2,646,885 3,074,464 3,299,992

Selected Quarterly Financial Da

Selected Quarterly Financial Data (Unaudited)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Selected Quarterly Financial Data (Unaudited)	13. Selected Quarterly Financial Data (unaudited) The following table presents selected unaudited financial data for each of the eight quarters in the two-year period ended December 31, 2018. The Company believes this information reflects all recurring adjustments necessary to fairly state this information when read in conjunction with the Company's financial statements and the related notes. Net loss per common share, basic and diluted, for the four quarters of each fiscal year may not sum to the total for the fiscal year because of the different number of shares outstanding during each period. The results of operations for any quarter are not necessarily indicative of the results to be expected for any future period (in thousands of dollars, except for share and per share data): Quarter Ended March 31 June 30 September 30 December 31 2018: Total revenues $ 30,565 $ 35,469 $ 38,104 $ 43,155 Gross profit 21,954 25,979 28,155 32,626 Net loss (11,117 ) (12,206 ) (10,244 ) (14,713 ) Net loss per common share, basic and diluted $ (0.47 ) $ (0.51 ) $ (0.43 ) $ (0.61 ) 2017: Total revenues $ 21,437 $ 23,854 $ 25,035 $ 28,183 Gross profit 15,100 17,110 18,115 20,476 Net loss (5,303 ) (6,444 ) (6,524 ) (11,149 ) Net loss per common share, basic and diluted $ (0.24 ) $ (0.29 ) $ (0.29 ) $ (0.48 )

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2018
Subsequent Events [Abstract]
Subsequent Events	14. Subsequent Events Lincolnshire Lease On February 22, 2019, the Company entered into the Third Amendment to the Sublease with Lincolnshire Property Holdings, LLC (“Third Amended Lincolnshire Lease”), to extend the term of its existing leased facility in Lincolnshire, Illinois. Pursuant to the Third Amended Lincolnshire Lease, the Company has the right to extend the Sublease for two additional terms and each term shall be for two years. The first additional term begins on November 1, 2019, and was exercised upon the execution of the agreement. In addition, the landlord will provide a tenant improvement allowance of up to $60,000. Base annual rent for the first extension term is estimated to be approximately $700,000 per year.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2018
Accounting Policies [Abstract]
Basis of Presentation	Basis of Presentation The accompanying consolidated financial statements have been prepared in accordance with U.S. generally accepted accounting principles and include the accounts of iRhythm Technologies, Inc. and its wholly-owned subsidiary, iRhythm Technologies Ltd., established in March 2016. All intercompany accounts and transactions have been eliminated. The financial statements of iRhythm Technologies Ltd. use the U.S. dollar as the functional currency
Use of Estimates	Use of Estimates The preparation of the consolidated financial statements in conformity with U.S. GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities as of the date of the financial statements and the reported amounts of revenues and expenses during the reporting period. On an ongoing basis, management evaluates its estimates, including those related to revenue recognition, contractual allowances for revenue, allowance for doubtful accounts, the useful lives of property and equipment, the recoverability of long-lived assets including the estimated usage of the printed circuit board assemblies (“PCBAs”), accounting for income taxes, the fair value of the Company’s common stock and stock-based compensation. The Company bases these estimates on historical and anticipated results, trends, and various other assumptions that the Company believes are reasonable under the circumstances, including assumptions as to future events. Actual results may differ from those estimates.
Fair Value of Financial Instruments	Fair Value of Financial Instruments The carrying amounts of certain of the Company’s financial instruments, which include cash equivalents, short-term investments, long-term investments, accounts receivable, accounts payable and accrued liabilities, approximate fair value due to their short maturities.
Cash Equivalents	Cash Equivalents Cash equivalents consist of short-term, highly liquid investments with original maturities of three months or less from the date of purchase.
Investments	Investments Short-term investments consist of debt securities classified as available-for-sale and have maturities greater than 90 days, but less than one year as of the balance sheet date. Long-term investments have maturities greater than one year as of the balance sheet date. All investments are carried at fair value based upon quoted market prices. Unrealized gains and losses on available-for-sale securities are excluded from earnings and are reported as a component of accumulated other comprehensive loss. The cost of available-for-sale securities sold is based on the specific-identification method. Realized gains and losses are included in earnings, and are derived for specific-identification method for determining the costs of investments sold. Amortization of premiums and accretion of discounts are reported as a component of interest income and interest expense, respectively.
Accounts Receivable, Allowance for Doubtful Accounts and Contractual Allowance	Accounts Receivable, Allowance for Doubtful Accounts and Contractual Allowance Accounts receivable consists of amounts due to the Company from institutions and third-party government and commercial payors and their related patients, as a result of the Company’s normal business activities. Accounts receivable is reported on the balance sheets net of an estimated allowance for doubtful accounts and contractual allowance. The Company establishes an allowance for doubtful accounts for estimated uncollectible receivables based on its historical experience and recognizes the provision as a component of selling, general and administrative expenses. The Company establishes a contractual allowance when it estimates that consideration to be received will be lower than the contracted rate based on its historical experience and recognizes the provision as a reduction to revenue. The following table presents the changes in the allowance for doubtful accounts (in thousands): Year ended December 31, 2018 2017 Balance, beginning of year $ 3,568 $ 1,792 Add: provision for doubtful accounts 5,826 3,640 Less: write-offs, net of recoveries and other adjustments (4,543 ) (1,864 ) Balance, end of year $ 4,851 $ 3,568 The following table presents the changes in the contractual allowance (in thousands): Year ended December 31, 2018 2017 Balance, beginning of year $ 7,444 $ 2,340 Add: allowance for contractual adjustments 9,392 5,763 Less: contractual adjustments (6,235 ) (659 ) Balance, end of year $ 10,601 $ 7,444 The following table presents the impact of allowance for doubtful accounts and contractual allowance on accounts receivable (in thousands): Year ended December 31, 2018 2017 Gross accounts receivable $ 37,429 $ 23,965 Less: allowance for doubtful accounts (4,851 ) (3,568 ) Less: contractual allowance (10,601 ) (7,444 ) Net accounts receivable $ 21,977 $ 12,953 The Company reviews and updates its estimates for the allowances for doubtful accounts and contractual allowance periodically to reflect its experience regarding historical collections. If management were to make different judgments or utilize different estimates in the allowances for doubtful accounts and contractual allowance, differences in the amount of reported selling, general and administrative expenses and revenue could result, respectively.
Concentration of Credit Risk	Concentrations of Risk Credit Risk Financial instruments that potentially subject the Company to a concentration of credit risk consist primarily of cash and cash equivalents, investments and accounts receivable. Cash, cash equivalents, and investments are deposited in financial institutions which, at times, may be in excess of federally insured limits. Cash equivalents are invested in highly rated money market funds. The Company invests in a variety of financial instruments, such as, but not limited to, United States Government securities, corporate notes, commercial paper and, by policy, limits the amount of credit exposure with any one financial institution or commercial issuer. The Company has not experienced any material losses on its deposits of cash and cash equivalents or investments. Concentrations of credit risk with respect to accounts receivable are limited due to the large number of customers comprising the Company’s customer base and their dispersion across many geographies. The Company does not require collateral. The Company records an allowance for doubtful accounts when it becomes probable that a receivable will not be collected. Federal government agencies, including CMS and the military, accounted for approximately 37%, 37% and 40% of the Company’s revenue for the years ended December 31, 2018, 2017 and 2016, respectively. Accounts receivable related to federal government agencies accounted for 18% and 27% at December 31, 2018 and 2017, respectively.
Concentration of Supply Risk	Supply Risk While the Company has not experienced manufacturing supply disruptions to date, the
Inventory	Inventory Inventory is stated at the lower of cost or net realizable value, cost being determined on a standard cost basis for material costs and on actual cost basis for labor and overhead, which approximates actual cost on a first in, first out (“FIFO”) basis, and market being determined as the lower of cost or net realizable value. The Company records write-downs of inventory that is obsolete or in excess of anticipated demand. The Company also records market value based write-downs on consideration of product lifecycle stage, technology trends, product development plans and assumptions about future demand and market conditions. Actual demand may differ from forecasted demand, and such differences may have a material effect on recorded inventory values. Inventory write-downs are charged to cost of revenue and establish a new cost basis for the inventory.
Property and Equipment	Property and Equipment Property and equipment are stated at cost less accumulated depreciation and amortization. Depreciation and amortization is computed using the straight-line method over the estimated useful lives of the assets, ranging from three to five years. Leasehold improvements are amortized over the shorter of the lease term or the estimated useful lives of the assets. Maintenance and repairs are charged to expense as incurred, and improvements and betterments are capitalized.
Internal-Use Software	Internal-Use Software The Company capitalizes costs related to internal-use software during the application development stage. Costs related to planning and post implementation activities are expensed as incurred. Capitalized internal-use software is amortized, and recognized as cost of revenue, on a straight-line basis over the estimated useful life, which is up to five years. The Company evaluates the useful lives of these assets on an annual basis, and tests for impairment whenever events or changes in circumstances occur that could impact the recoverability of these assets. Capitalized internal-use software costs are classified as a component of property and equipment.
Goodwill	Goodwill Goodwill represents the excess of the purchase price paid over the fair value of tangible and identifiable intangible net assets acquired in business combinations. Goodwill is tested for impairment on an annual basis and at any other time if events occur or circumstances indicate that the carrying amount of goodwill may not be recoverable. Such events or circumstances may include significant adverse changes in the general business climate, among other things. The impairment test is performed by determining the enterprise fair value of the Company, which is primarily based on the Company’s market capitalization. If the Company’s carrying value, as a one reporting unit entity, is less than its fair value, then the fair value is allocated to all of its assets and liabilities (including any unrecognized intangible assets) as if the fair value was the purchase price to acquire the Company. The excess of the fair value over the amounts assigned to the Company’s assets and liabilities is the implied fair value of the goodwill. If the carrying amount of goodwill exceeds the implied fair value of that goodwill, an impairment loss is recognized in an amount equal to that excess. The Company performs its annual evaluation of goodwill during the fourth quarter of each fiscal year. The Company did not record any charges related to goodwill impairment in any of the periods presented in these consolidated financial statements.
Impairment of Long-Lived Assets	Impairment of Long-Lived Assets The Company annually reviews long-lived assets for impairment or whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability is measured by comparison of the carrying amount to the future net cash flows which the assets are expected to generate. If such assets are considered to be impaired, the impairment to be recognized is measured as the amount by which the carrying amount of the assets exceeds the projected discounted future net cash flows arising from the asset. To date, there have been no such impairments of long-lived assets.
Other Assets	Other Assets Included in the other assets are printed circuit board assemblies (“PCBAs”) totaling $2.5 million and $3.3 million as of December 31, 2018 and 2017, respectively. The Company uses a PCBA in each wearable Zio XT monitor which is used numerous times and has a useful life beyond one year. Each time the PCBA is used in a wearable Zio XT monitor, a portion of the cost of the PCBA is recorded as a cost of revenue. The Company has based its estimates of how many times a PCBA can be used on testing in research and development, loss rates, product obsolescence, and the amount of time it takes the device to go through the manufacturing, shipping, customer shelf and patient wear time and upload process. The Company periodically evaluates the use estimate.
Comprehensive Loss	Comprehensive Loss Comprehensive loss represents all changes in stockholders’ equity during the period from non-owner sources. The Company’s unrealized gains and losses on available-for-sale securities represent the only component of other comprehensive income (loss) that are excluded from the reported net loss and that are presented in the consolidated statements of comprehensive income.
Revenue Recognition	Revenue Recognition The Company adopted Accounting Standard Codification Topic 606, Revenue from Contracts with Customers Health Care Entities - Revenue Recognition Revenue Recognition The Company’s revenue is generated primarily from the provision of its cardiac rhythm monitoring service, the Zio XT service. The Zio XT is a cardiac rhythm monitoring service that has a patient wear period of up to 14 days and is billable when the monitoring reports are delivered to the healthcare provider, which is also when the service is complete and the Company recognizes revenue. The time from when the patient has the Zio XT device applied to the time the report is posted is generally around 20 days. The Company accounts for contract revenue with a customer when there is a legally enforceable contract between the Company and the customer, the rights of the parties are identified, the contract has commercial substance, and collectability of the contract consideration is probable. The Company's revenue is measured based on consideration specified in the contract with each customer. A unique aspect of healthcare is the involvement of multiple parties to the service transaction. In addition to the patient, often a third-party, for example a commercial or governmental payor or healthcare institution, like a hospital or clinic, will pay the Company for some or all of the service on the patient’s behalf. Separate contractual arrangements exist between the Company and third-party payors that establish amounts the third-party payor will pay on behalf of a patient for covered services rendered and should be considered in determining collectability and the transaction price for services provided to a patient covered by that third-party payor. The Company recognizes revenue on an accrual basis based on estimates of the amount that will ultimately be realized. These estimates require significant judgment by management. In determining the amount to recognize for a delivered report, the Company considers factors such as claim payment history from both payors and patient out-of-pocket costs, payor coverage, whether there is a contract between the payor or healthcare institution and the Company, historical amount received for the service, and any current developments or changes that could impact reimbursement and healthcare institution payments. A summary of the payment arrangements with third-party payors and healthcare institutions is as follows: • Contracted third-party payors – The Company has contracts with negotiated prices for services provided for patients with commercial healthcare insurance carriers • Centers for Medicare and Medicaid Services (“CMS”) – The Company has received independent diagnostic testing facility approval from regional Medicare Administrative Contractors and will receive reimbursement per the relevant Current Procedural Terminology (“CPT”) code rate for the services rendered to the patient covered by CMS. • Non-contracted third-party payors – Non-contracted commercial and government payors often reimburse out-of-network rates provided under the relevant CPT codes on a case-by-case basis. The transaction price is based on factors including an average of the Company’s historical collection experience for its non-contracted services. This rate is reviewed at least quarterly. • Healthcare institutions – Healthcare institutions are typically hospitals or physician practices in which the Company has negotiated amounts for its monitoring services, including certain governmental agencies such as the Veteran’s Administration and Department of Defense. The Company is utilizing the portfolio approach practical expedient under ASC 606 for revenue recognition. The Company accounts for the contracts within each portfolio as a collective group, rather than individual contracts. Based on history with these portfolios and the similar nature and characteristics of the patients within each portfolio, the Company has concluded that the financial statement effects are not materially different than if accounting for revenue on a contract-by-contract basis. For the healthcare institution and CMS portfolios, the Company has historical experience of collecting substantially all of the negotiated contractual rates and determined at contract inception that these customers, and or their related third-party payor that pays the Company on their behalf, have the intention and ability to pay the promised consideration. As such, the Company is not providing an implicit price concession but, rather, has chosen to accept the risk of default, and any subsequent impairment of the related receivable are recorded as bad debt expense. For contracted portfolios, the Company is providing an implicit price concession because, while the Company has a contract with the underlying payor, the Company expects to accept a lower amount of consideration when claims are adjudicated and allowable claims are determined by the commercial payor. The implicit price concession is recorded as variable consideration to the transaction price and recorded as an adjustment to revenue as a contractual allowance. Historical cash collection indicates that it is probable that substantially all of the allowable claim amount will be received, and hence this amount is recorded as revenue. Any subsequent impairment of the related receivable is recorded as bad debt expense. For non-contracted portfolios, the Company is providing an implicit price concession because the Company does not have a contract with the underlying payor, the result of which requires the Company to estimate transaction price based on historical cash collections utilizing the expected value method. Subsequent adjustments to the transaction price are recorded as an adjustment to revenue and not as bad debt expense. Disaggregation of Revenue The Company disaggregates revenue from contracts with customers by payor type. The Company believes these categories aggregate the payor types by nature, amount, timing and uncertainty of its revenue streams. Disaggregated revenue by payor type and major service line for the year ended December 31, 2018 was as follows (in thousands): Year Ended December 31, 2018 Commercial Payors $ 69,363 Centers for Medicare & Medicaid 40,552 Healthcare Institutions 37,378 Total $ 147,293
Contract Liabilities	Contract Liabilities ASC 606 requires an entity to present a revenue contract as a contract liability when the Company has an obligation to transfer goods or services to a customer for which the Company has received consideration from the customer, or an amount of consideration from the customer is due and unconditional (whichever is earlier). Certain of the Company’s customers pay the Company directly for the Zio XT service upon shipment of devices. Such advance payments, or contract liabilities, are recorded as deferred revenue on the Condensed Consolidated Balance Sheets and revenue is recognized when reports are delivered to physicians. During the year ended December 31, 2018, $1.2 million relating to the contract liability balance at the beginning of 2018 was recognized as revenue.
Contract Costs	Contract Costs Under ASC 340, the incremental costs of obtaining a contract with a customer are recognized as an asset. Incremental costs of obtaining a contract are those costs that an entity incurs to obtain a contract with a customer that it would not have incurred if the contract had not been obtained. The Company’s current commission programs are considered incremental. However, as a practical expedient, ASC 340 permits the Company to immediately expense contract acquisition costs, as the asset that would have resulted from capitalizing these costs will be amortized in one year or less.
Cost of Revenue	Cost of Revenue Cost of revenue is expensed as incurred, and includes direct labor, material costs, equipment and infrastructure expenses, amortization of internal-use software, allocated overhead, and shipping and handling. Direct labor includes payroll and personnel-related costs involved in manufacturing, data analysis, and customer service. Material costs include both the disposable costs of the device and amortization of the PCBAs. Each time the PCBA is used in a wearable Zio XT monitor, a portion of the cost of the PCBA is recorded as a cost of revenue.
Research and Development	Research and Development The Company’s research and development costs are expensed as incurred. Research and development costs include, but are not limited to, payroll and personnel-related expenses, laboratory supplies, consulting costs and overhead charges.
Income Taxes	Income Taxes The Company uses the asset and liability method to account for income taxes in accordance with the authoritative guidance for income taxes. Under this method, deferred tax assets and liabilities are determined based on future tax consequences attributable to differences between the financial statement carrying amounts of existing assets and liabilities and their respective tax bases, and tax loss and credit carryforwards. Deferred tax assets and liabilities are measured using enacted tax rates applied to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect on deferred tax assets and liabilities of a change in tax rates is recognized in income in the period that includes the enactment date. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized. On December 22, 2017, the 2017 Tax Cuts and Jobs Act (Tax Act) was enacted into law and the new legislation contains several key tax provisions that affected the Company, including a one-time mandatory transition tax on accumulated foreign earnings and a reduction of the corporate income tax rate to 21% effective January 1, 2018, among others. The Company was required to recognize the effect of the tax law changes in the period of enactment, such as determining the transition tax and remeasuring U.S. deferred tax assets and liabilities. In December 2017, the SEC staff issued Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act (“SAB 118”), which allowed the Company to record provisional amounts during a measurement period not to extend beyond one year of the enactment date. As a result, the Company previously provided a provisional estimate of the effect of the Tax Act in its financial statements. In the fourth quarter of 2018, the Company completed the analysis to determine the effect of the Tax Act and recorded immaterial adjustments as of December 31, 2018. The Company recognizes the effect of income tax positions only if those positions are more likely than not of being sustained. Recognized income tax positions are measured at the largest amount that has a greater than 50% likelihood of being realized. Changes in recognition or measurement are reflected in the period in which the change in judgment occurs. The Company records interest and penalties related to unrecognized tax benefits in income tax expense. To date, there have been no interest or penalties charged in relation to the unrecognized tax benefits.
Stock-based Compensation	Stock-based Compensation The Company measures its stock-based awards made to employees based on the estimated fair values of the awards as of the grant date. The fair value of stock options is determined using the Black-Scholes option pricing model. Stock-based compensation expense is recognized over the requisite service period using the straight-line method and is based on the value of the portion of stock-based payment awards that is ultimately expected to vest. As such, the Company’s stock-based compensation is reduced for the estimated forfeitures at the date of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. For restricted stock, the compensation cost for these awards is based on the closing price of the Company’s common stock on the date of grant, and recognized as compensation expense on a straight-line basis over the requisite service period. The Company recognizes compensation expense related to the Employee Stock Purchase Program (“ESPP”) based on the estimated fair value of the options on the date of grant, net of estimated forfeitures. The Company estimates the grant date fair value, and the resulting stock-based compensation expense, using the Black-Scholes option pricing model for each purchase period. The grant date fair value is expensed on a straight-line basis over the offering period.
Net Loss per Common Share	Net Loss per Common Share Basic net loss per common share is calculated by dividing the net loss by the weighted average number of shares of common stock outstanding during the period, without consideration of potentially dilutive securities. Diluted net loss per common share is the same as basic net loss per common share for all periods presented, since the effect of potentially dilutive securities are anti-dilutive.
Recent Accounting Guidance / Pronouncements	Recently Adopted Accounting Guidance In May 2014, the Financial Accounting Standards Board (“FASB”), issued Accounting Standards Update (“ASU”) No. 2014-09, Revenue from Contracts with Customers (Topic 606) The Company adopted this standard on January 1, 2018 and used the modified retrospective approach. Upon adoption, the Company recognized the cumulative effect of $1.4 million as an adjustment to decrease the opening balance of the Company’s accumulated deficit. This adjustment did not have a material impact on the Company’s consolidated financial statements. Prior periods were not retrospectively adjusted. The following table presents the impact of adoption of ASU 2014-09 on the Consolidated Statement of Operations and the Consolidated Balance Sheet (in thousands): Year Ended December 31, 2018 Consolidated Statement of Operations Impact: As Reported Without the adoption of Topic 606 Impact Revenue $ 147,293 $ 145,320 $ 1,973 Sales, General & Administrative Expense $ 131,582 $ 132,603 $ (1,021 ) Net Loss $ (48,280 ) $ (51,274 ) $ 2,994 December 31, 2018 Consolidated Balance Sheet Impact: As Reported Without the adoption of Topic 606 Impact Accounts Receivable, net $ 21,977 $ 17,629 $ 4,348 Accumulated Deficit $ (203,515 ) $ (207,863 ) $ 4,348 In November 2016, the FASB issued ASU No. 2016-18 Statement of Cash Flow: Restricted Cash (Topic 230), which provides guidance on the classification of restricted cash to be included with cash and cash equivalents when reconciling the beginning of period and end of period total amounts on the statement of cash flows. The amendments of this ASU are effective for interim and annual periods beginning after December 15, 2017. The standard must be applied retrospectively to all periods presented and the Company adopted the standard on January 1, 2018. The adoption of this standard did not have a material impact on the Company’s Condensed Consolidated Statement of Cash Flows. Recent Accounting Pronouncements Not Yet Adopted In February 2016, the FASB issued ASU No. 2016-02, Leases (“Topic 842”), which requires lessees to recognize lease liabilities and corresponding right-of-use assets on the Consolidated Balance Sheet for all leases. For finance leases, the lessee would recognize interest expense and amortization of the right-of-use asset and, for operating leases, the lessee would recognize a straight-line lease expense. It also changes the definition of a lease and expands the disclosure requirements of lease arrangements. The Company is currently evaluating the impact of the adoption of this standard, and expects that the impact on the Consolidated Balance Sheet will be material as a result of recognizing right of use assets and liabilities for existing real estate leases. The Company has no embedded leases with suppliers. In addition, the standard will require changes to the Company’s current accounting policies, processes, and internal controls. The Company expects to adopt the following practical expedients allowed under Topic 842: • The package of three practical expedients, which allows entities to make an election that allows them not to reassess (1) embedded leases not classified as leases under Topic 842, (2) Lease classification of existing or expiring leases, and (3) indirect costs for existing or expired leases. • Combining lease and non-lease components practical expedient, which allows lessees, as an accounting policy election by class of underlying asset, to choose not to separate non-lease components from lease components and instead to account for each separate lease component and the non-lease components associated with that lease component as a single lease component. • Comparative reporting practical expedient, which allows entities to initially apply Topic 842 at the adoption date and recognize a cumulative-effect adjustment to the opening balance of retained earnings in the period of adoption. The Company will adopt Topic 842 in the first quarter of fiscal

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2018
Schedule of Changes in Allowance for Doubtful Accounts	The following table presents the changes in the allowance for doubtful accounts (in thousands): Year ended December 31, 2018 2017 Balance, beginning of year $ 3,568 $ 1,792 Add: provision for doubtful accounts 5,826 3,640 Less: write-offs, net of recoveries and other adjustments (4,543 ) (1,864 ) Balance, end of year $ 4,851 $ 3,568
Schedule of Changes in Contractual Allowance	The following table presents the changes in the contractual allowance (in thousands): Year ended December 31, 2018 2017 Balance, beginning of year $ 7,444 $ 2,340 Add: allowance for contractual adjustments 9,392 5,763 Less: contractual adjustments (6,235 ) (659 ) Balance, end of year $ 10,601 $ 7,444
Schedule of Impact of Allowance for Doubtful Accounts and Contractual Allowance on Accounts Receivable	The following table presents the impact of allowance for doubtful accounts and contractual allowance on accounts receivable (in thousands): Year ended December 31, 2018 2017 Gross accounts receivable $ 37,429 $ 23,965 Less: allowance for doubtful accounts (4,851 ) (3,568 ) Less: contractual allowance (10,601 ) (7,444 ) Net accounts receivable $ 21,977 $ 12,953
Disaggregated Revenue by Payor Type and Major Service	The Company disaggregates revenue from contracts with customers by payor type. The Company believes these categories aggregate the payor types by nature, amount, timing and uncertainty of its revenue streams. Disaggregated revenue by payor type and major service line for the year ended December 31, 2018 was as follows (in thousands): Year Ended December 31, 2018 Commercial Payors $ 69,363 Centers for Medicare & Medicaid 40,552 Healthcare Institutions 37,378 Total $ 147,293
ASU 2014-09
Schedule of Impact of Adoption of ASU 2014-09 on Financial Statements	The following table presents the impact of adoption of ASU 2014-09 on the Consolidated Statement of Operations and the Consolidated Balance Sheet (in thousands): Year Ended December 31, 2018 Consolidated Statement of Operations Impact: As Reported Without the adoption of Topic 606 Impact Revenue $ 147,293 $ 145,320 $ 1,973 Sales, General & Administrative Expense $ 131,582 $ 132,603 $ (1,021 ) Net Loss $ (48,280 ) $ (51,274 ) $ 2,994 December 31, 2018 Consolidated Balance Sheet Impact: As Reported Without the adoption of Topic 606 Impact Accounts Receivable, net $ 21,977 $ 17,629 $ 4,348 Accumulated Deficit $ (203,515 ) $ (207,863 ) $ 4,348

Cash Equivalents and Investme_2

Cash Equivalents and Investments (Tables)	12 Months Ended
	Dec. 31, 2018
Cash Equivalents And Investments [Abstract]
Schedule of Fair Value of Securities, not Including Cash	The fair value of securities, not including cash at December 31, 2018 and 2017 were as follows (in thousands): December 31, 2018 Amortized Gross Unrealized Estimated Cost Gains Losses Fair Value Money market funds $ 10,606 $ — $ — $ 10,606 U.S. government securities 9,976 — (1 ) 9,975 Corporate notes 16,514 3 (18 ) 16,499 Commercial paper 36,331 — — 36,331 Total available-for-sale marketable debt securities $ 73,427 $ 3 $ (19 ) $ 73,411 Classified as: Cash equivalents $ 15,091 Short-term investments 58,320 Total $ 73,411 December 31, 2017 Amortized Gross Unrealized Estimated Cost Gains Losses Fair Value Money market funds $ 5,574 $ — $ — $ 5,574 U.S. government securities 24,969 — (29 ) 24,940 Corporate notes 24,539 — (36 ) 24,503 Commercial paper 47,243 — — 47,243 Total available-for-sale marketable debt securities $ 102,325 $ — $ (65 ) $ 102,260 Classified as: Cash equivalents $ 5,574 Short-term investments 93,692 Long-term investments 2,994 Total $ 102,260
Schedule of Fair Value of Short-term and Long-term Marketable Securities Classified by Maturity	As the carrying value approximates the fair value for the Company’s cash equivalents, short-term and long-term marketable securities shown in the tables above, the following table summarizes the fair value of the Company’s cash equivalents, short-term and long-term marketable securities classified by maturity as of December 31, 2018 and 2017 (in thousands): December 31, 2018 2017 Due within one year $ 73,411 $ 99,266 Due after one year through three years — 2,994 Total available-for-sale marketable debt securities $ 73,411 $ 102,260
Schedule of Available-for-Sale Securities Unrealized Loss Position	The following tables present the Company's available-for-sale securities that were in an unrealized loss position as of December 31, 2018 and December 31, 2017 (in thousands): December 31, 2018 Less than 12 months 12 Months or Greater Assets Fair Value Unrealized Loss Fair Value Unrealized Loss U.S. government securities $ 5,977 $ (1 ) $ — $ — Corporate notes 11,521 (10 ) 2,993 (8 ) Total $ 17,498 $ (11 ) $ 2,993 $ (8 ) December 31, 2017 Less than 12 months Assets Fair Value Unrealized Loss U.S. government securities $ 24,940 $ (29 ) Corporate notes 24,503 (36 ) Commercial paper 7,866 — Total $ 57,309 $ (65 )

Fair Value Measurements (Tables

Fair Value Measurements (Tables)	12 Months Ended
	Dec. 31, 2018
Fair Value Disclosures [Abstract]
Schedule of Fair Value of Company's Financial Assets and Liabilities	The following table presents the fair value of the Company’s financial assets determined using the inputs defined above (in thousands). December 31, 2018 Level 1 Level 2 Level 3 Total Assets Money market funds $ 10,606 $ — $ — $ 10,606 U.S. government securities — 9,975 — 9,975 Corporate notes — 16,499 — 16,499 Commercial paper — 36,331 — 36,331 Total $ 10,606 $ 62,805 $ — $ 73,411 December 31, 2017 Level 1 Level 2 Level 3 Total Assets Money market funds $ 5,574 $ — $ — $ 5,574 U.S. government securities — 24,940 — 24,940 Corporate notes — 24,503 — 24,503 Commercial paper — 47,243 — 47,243 Total $ 5,574 $ 96,686 $ — $ 102,260

Balance Sheet Components (Table

Balance Sheet Components (Tables)	12 Months Ended
	Dec. 31, 2018
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Components of Inventory and Printed Circuit Board Assemblies ("PCBAs")	Inventory and Printed Circuit Board Assemblies (“PCBAs”), which are recorded as other assets as they are used many times over a period longer than one year on average, consisted of the following (in thousands): December 31, 2018 2017 Raw materials $ 1,028 $ 1,103 Finished goods 3,565 3,830 Total $ 4,593 $ 4,933 December 31, 2018 2017 Reported on the consolidated balance sheet as: Inventory $ 2,062 $ 1,683 Other assets 2,531 3,250 Total $ 4,593 $ 4,933
Components of Property and Equipment, Net	Property and equipment, net consisted of the following (in thousands): December 31, 2018 2017 Laboratory and manufacturing equipment $ 2,750 $ 1,994 Computer equipment and software 1,062 1,015 Furniture and fixtures 925 953 Leasehold improvements 726 726 Internal-use software 8,925 4,598 Total property and equipment, gross 14,388 9,286 Less: accumulated depreciation and amortization (5,230 ) (3,065 ) Total property and equipment, net $ 9,158 $ 6,221
Components of Accrued Liabilities	Accrued liabilities consisted of the following (in thousands): December 31, 2018 2017 Accrued vacation $ 2,825 $ 2,081 Accrued payroll and related expenses 18,188 9,361 Accrued professional services fees 1,243 1,041 Claims payable 2,374 1,375 Other 1,940 1,786 Total accrued liabilities $ 26,570 $ 15,644

Commitments and Contingencies (

Commitments and Contingencies (Tables)	12 Months Ended
	Dec. 31, 2018
Commitments And Contingencies Disclosure [Abstract]
Schedule of Future Minimum Lease Payments Under Non-cancelable Operating Leases	The following table summarizes the Company’s future minimum lease payments as of December 31, 2018 (in thousands): Year Ending December 31: 2019 $ 8,135 2020 10,669 2021 10,828 2022 11,150 2023 11,483 Thereafter 98,209 Total $ 150,474

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2018
Debt Disclosure [Abstract]
Schedule of Future Minimum Payments	Future minimum payments under the Third Amended and Restated Loan and Security Agreement with Silicon Valley Bank at December 31, 2018 are as follows (in thousands): Year Ending December 31: 2019 $ 1,662 2020 3,603 2021 12,983 2022 12,429 2023 9,934 Total 40,611 Less: Amount representing interest (5,611 ) Less: Amount representing debt issuance costs (101 ) Total carrying value $ 34,899

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2018
Income Tax Disclosure [Abstract]
Components of Provision for Income Taxes	There was no provision for the years ended December 31, 2017 and 2016. The following table presents components of the Company’s provision for income taxes as of December 31, 2018 (in thousands): Year Ended December 31, 2018 Current: Federal $ — State — Foreign 80 Total current tax expense 80 Deferred: Federal — State — Foreign (36 ) Total deferred tax benefit (36 ) Provision for income taxes $ 44
Reconciliation of Tax Expense Computed at Statutory Federal Rate and Company's Tax Expense	The following table presents a reconciliation of the tax expense computed at the statutory federal rate and the Company’s tax expense for the period presented (in thousands): Year Ended December 31, 2018 2017 2016 Tax at statutory federal rate (10,143 ) (10,034 ) $ (7,107 ) Stock-based compensation (8,562 ) (7,634 ) 255 Meals and Entertainment 309 198 134 Other 149 (145 ) 782 Tax credits (1,015 ) (644 ) (139 ) 2017 Tax Act 44 21,928 — Change in valuation allowance 19,262 (3,669 ) 6,075 Provision for income taxes $ 44 $ — $ — ________________ (1)
Significant Components of Deferred Tax Assets and Liabilities	Significant components of the Company’s deferred tax assets and liabilities are as follows (in thousands): December 31, 2018 2017 Deferred tax assets: Net operating loss carryforwards $ 50,334 $ 35,338 Tax credit carryforwards 4,287 2,642 Allowances and other 11,694 6,682 Depreciation and amortization (388 ) (414 ) Total deferred tax assets 65,927 44,248 Valuation allowance (65,886 ) (44,248 ) Net deferred tax assets $ 41 $ —
Summary of Valuation Allowance for Deferred Tax Assets	The valuation allowance for deferred tax assets consisted of the following activity for the years ended December 31, 2018, 2017 and 2016 (in thousands): Balance at beginning of year Additions Deductions Balance at end of year Year ended December 31, 2016 $ 38,565 $ 6,296 $ — $ 44,861 Year ended December 31, 2017 44,861 — 613 44,248 Year ended December 31, 2018 $ 44,248 $ 21,638 $ — $ 65,886
Reconciliation of Unrecognized Tax Benefit	A reconciliation of the Company’s unrecognized tax benefit amount is as follows (in thousands): Year Ended December 31, 2018 2017 2016 Balance at beginning of year $ 943 $ 616 $ 570 Additions for tax positions taken in current year 441 328 75 Increases in balance related to prior year tax positions 75 — — Decreases in balances related to prior year tax position — (1 ) (29 ) Balance at end of year $ 1,459 $ 943 $ 616

Stockholder's Equity (Tables)

Stockholder's Equity (Tables)	12 Months Ended
	Dec. 31, 2018
Equity [Abstract]
Schedule of Common Stock Shares Reserved for Future Issuance	The Company had reserved shares of common stock for issuance as follows: December 31, 2018 2017 Options issued and outstanding 2,094,137 2,601,181 Unvested restricted stock units 547,891 468,426 Common stock warrants issued and outstanding 4,857 4,857 Shares available for grant under future stock plans 5,607,014 4,650,669 Total 8,253,899 7,725,133

Stock Incentive Plans (Tables)

Stock Incentive Plans (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Summary of Share-based Awards Available for Grant under 2016 Plan	A summary of share-based awards available for grant under the 2016 Plan is as follows: Shares for Grant Balance at December 31, 2015 331,938 Additional options authorized 3,865,000 Awards granted (466,914 ) Awards forfeited 13,013 Balance at December 31, 2016 3,743,037 Additional awards authorized 1,106,966 Awards granted (837,436 ) Awards forfeited 21,585 Balance at December 31, 2017 4,034,152 Additional awards authorized 1,168,865 Awards granted (666,913 ) Awards forfeited 124,478 Awards withheld for tax purposes 56,710 Balance at December 31, 2018 4,717,292
Summary of Stock Option Activity Under 2006 and 2016 Plans, Including Grants To Nonemployees	The following table summarizes stock option activity under the 2006 and 2016 Plans, including grants to nonemployees: Options Outstanding Options Outstanding Weighted- Average Exercise Price Per Share Weighted- Average Remaining Contractual Life (years) Aggregate Intrinsic Value (in Balance at December 31, 2015 2,685,913 $ 4.81 7.63 $ 11,589 Options granted 361,385 $ 15.65 Options exercised (57,067 ) $ 2.33 Options forfeited (13,013 ) $ 6.92 Balances at December 31, 2016 2,977,218 $ 6.16 6.93 $ 70,979 Options granted 465,271 $ 36.76 Options exercised (827,556 ) $ 4.11 Options forfeited (13,752 ) $ 14.43 Balance at December 31, 2017 2,601,181 $ 12.24 7.17 113,958 Options granted 366,928 $ 68.32 Options exercised (798,424 ) $ 7.19 Options forfeited (75,548 ) $ 34.30 Balance at December 31, 2018 2,094,137 $ 23.20 7.02 97,976 Options exercisable – December 31, 2018 1,278,367 $ 10.27 6.18 75,695 Options vested and expected to vest – December 31, 2018 2,050,504 $ 22.60 6.99 97,085
Summary of Non-vested Restricted Stock Units ("RSUs")	Shares Underlying RSUs Weighted Average Grant Date Fair Value Weighted Remaining Vesting Period (in years) Aggregate Intrinsic Value (in Non-vested as of December 31, 2017 468,426 $ 36.96 2.54 $ 26,255 Granted 299,985 $ 71.54 Vested and released (114,880 ) $ 31.53 Forfeited (105,640 ) $ 38.81 Non-vested as of December 31, 2018 547,891 $ 56.62 2.45 $ 38,067

Stock-Based Compensation (Table

Stock-Based Compensation (Tables)	12 Months Ended
	Dec. 31, 2018
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Schedule of Fair Value of Employee Stock Options Estimated Using Weighted Average Assumptions	The fair value of employee stock options was estimated using the weighted average assumptions below. Each of these inputs is subjective and its determination generally requires significant judgment. Year Ended December 31 2018 2017 2016 Expected term (in years) 6.1 6.1 6.1 Expected volatility 45.7 % 51.9 % 60.0 % Risk-free interest rate 2.75 % 2.07 % 1.42 % Dividend yield 0.0 % 0.0 % 0.0 %
Summary of Total Stock-Based Compensation Expense for Options, RSUs and ESPP Included in Consolidated Statements of Operations and Comprehensive Loss	The following table summarizes the total stock-based compensation expense for options, RSUs and ESPP included in the consolidated statements of operations for all periods presented (in thousands): Year Ended December 31 2018 2017 2016 Cost of revenue $ 193 $ 163 $ 17 Research and development 3,057 1,733 203 Selling, general and administrative 13,079 8,227 1,651 Total stock-based compensation expense $ 16,329 $ 10,123 $ 1,871

Net Loss Per Common Share (Tabl

Net Loss Per Common Share (Tables)	12 Months Ended
	Dec. 31, 2018
Earnings Per Share [Abstract]
Computation of Basic and Diluted Net Loss per Share	The following table sets forth the computation of the basic and diluted net loss per share during the years ended December 31, 2018, 2017 and 2016 (in thousands, except share and per share data): Year Ended December 31, 2018 2017 2016 Numerator: Net loss $ (48,280 ) $ (29,420 ) $ (20,903 ) Denominator: Weighted-average shares used to compute net loss per common share, basic and diluted 23,885,858 22,627,327 5,285,847 Net loss per common share, basic and diluted $ (2.02 ) $ (1.30 ) $ (3.95 )
Schedule of Anti-dilutive Securities Excluded from Diluted Net Loss per Common Share	The following outstanding shares of potentially dilutive securities have been excluded from diluted net loss per common share for the years ended December 31, 2018, 2017 and 2016 because their inclusion would be anti-dilutive: Year Ended December 31, 2018 2017 2016 Options to purchase common stock 2,094,137 2,601,181 2,977,218 RSUs issued and unvested 547,891 468,426 105,529 Warrants to purchase common stock 4,857 4,857 217,245 Total 2,646,885 3,074,464 3,299,992

Selected Quarterly Financial _2

Selected Quarterly Financial Data (Unaudited) (Tables)	12 Months Ended
	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Schedule of Quarterly Financial Information	The following table presents selected unaudited financial data for each of the eight quarters in the two-year period ended December 31, 2018. The Company believes this information reflects all recurring adjustments necessary to fairly state this information when read in conjunction with the Company's financial statements and the related notes. Net loss per common share, basic and diluted, for the four quarters of each fiscal year may not sum to the total for the fiscal year because of the different number of shares outstanding during each period. The results of operations for any quarter are not necessarily indicative of the results to be expected for any future period (in thousands of dollars, except for share and per share data): Quarter Ended March 31 June 30 September 30 December 31 2018: Total revenues $ 30,565 $ 35,469 $ 38,104 $ 43,155 Gross profit 21,954 25,979 28,155 32,626 Net loss (11,117 ) (12,206 ) (10,244 ) (14,713 ) Net loss per common share, basic and diluted $ (0.47 ) $ (0.51 ) $ (0.43 ) $ (0.61 ) 2017: Total revenues $ 21,437 $ 23,854 $ 25,035 $ 28,183 Gross profit 15,100 17,110 18,115 20,476 Net loss (5,303 ) (6,444 ) (6,524 ) (11,149 ) Net loss per common share, basic and diluted $ (0.24 ) $ (0.29 ) $ (0.29 ) $ (0.48 )

Organization and Description _2

Organization and Description of Business - Additional Information (Details) $ in Thousands	Oct. 31, 2016USD ($)shares	Oct. 19, 2016USD ($)shares	Oct. 05, 2016	Dec. 31, 2018	Dec. 31, 2016USD ($)
Class Of Stock [Line Items]
Reverse stock split description				On October 4, 2016, the Company’s board of directors approved an amendment to the Company’s amended and restated certificate of incorporation to effect a reverse split of the Company’s issued and outstanding common stock at a 1-for- 5.882698 ratio, which was effected on October 5, 2016.
Common stock, reverse stock split ratio			5.882698
Proceeds from stock issuance, net of underwriting discounts and commissions					$ 114,436
Carrying value of convertible preferred stock converted into common stock					$ 97,553
Initial Public Offering
Class Of Stock [Line Items]
Common stock issued | shares		7,238,235
Initial public offering closing date		Oct. 25, 2016
Proceeds from stock issuance, net of underwriting discounts and commissions		$ 110,700
Underwriting discounts and commissions		8,600
Other expenses		$ 3,700
Initial Public Offering | Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Number of shares converted into common stock | shares	13,375,333
Carrying value of convertible preferred stock converted into common stock	$ 97,600

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Schedule of Changes in Allowance for Doubtful Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Balance, beginning of year	$ 3,568	$ 1,792
Add: provision for doubtful accounts	5,826	3,640
Less: write-offs, net of recoveries and other adjustments	(4,543)	(1,864)
Balance, end of year	$ 4,851	$ 3,568

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Schedule of Changes in Contractual Allowance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Balance, beginning of year	$ 7,444	$ 2,340
Add: allowance for contractual adjustments	9,392	5,763
Less: contractual adjustments	(6,235)	(659)
Balance, end of year	$ 10,601	$ 7,444

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Schedule of Impact of Allowance for Doubtful Accounts and Contractual Allowance on Accounts Receivable (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Receivables [Abstract]
Gross accounts receivable	$ 37,429	$ 23,965
Less: allowance for doubtful accounts	(4,851)	(3,568)
Less: contractual allowance	(10,601)	(7,444)
Net accounts receivable	$ 21,977	$ 12,953

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018USD ($)Lease	Dec. 31, 2017USD ($)	Dec. 31, 2016	Jan. 01, 2018USD ($)
Summary Of Significant Accounting Policies [Line Items]
Impairment long-lived assets	$ 0
Contract liability balance, revenue recognized	$ 1,200,000
U.S. statutory tax rate	21.00%
Income tax position likely of being realized upon ultimate settlement	greater than 50%
Unrecognized tax benefits, income tax interest or penalties charge	$ 0
Cumulative effect of adopting new standard	$ (203,515,000)	$ (156,589,000)
Number of embedded leases with suppliers | Lease	0
Impact | ASU 2014-09
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect of adopting new standard	$ 4,348,000			$ 1,400,000
Internal-Use Software
Summary Of Significant Accounting Policies [Line Items]
Estimated useful life	5 years
Printed Circuit Board Assemblies
Summary Of Significant Accounting Policies [Line Items]
Total other assets	$ 2,500,000	$ 3,300,000
Minimum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	3 years
Maximum
Summary Of Significant Accounting Policies [Line Items]
Property and equipment, estimated useful life	5 years
Maximum | Zio XT service
Summary Of Significant Accounting Policies [Line Items]
Equipment wear period	14 days
Revenue | Customer Concentration Risk | Federal Government Agencies
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk	37.00%	37.00%	40.00%
Accounts Receivable | Accounts Receivable Concentration Risk | Federal Government Agencies
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk	18.00%	27.00%

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Disaggregated Revenue by Payor Type and Major Service (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disaggregation Of Revenue [Line Items]
Revenue	$ 43,155	$ 38,104	$ 35,469	$ 30,565	$ 28,183	$ 25,035	$ 23,854	$ 21,437	$ 147,293	$ 98,509	$ 64,072
Commercial Payors
Disaggregation Of Revenue [Line Items]
Revenue									69,363
Centers for Medicare and Medicaid
Disaggregation Of Revenue [Line Items]
Revenue									40,552
Healthcare Institutions
Disaggregation Of Revenue [Line Items]
Revenue									$ 37,378

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Schedule of Impact of Adoption of ASU 2014-09 on Financial Statements (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue	$ 43,155	$ 38,104	$ 35,469	$ 30,565	$ 28,183	$ 25,035	$ 23,854	$ 21,437	$ 147,293	$ 98,509	$ 64,072
Sales, General & Administrative Expense									131,582	84,737	51,621
Net Loss	(14,713)	$ (10,244)	$ (12,206)	$ (11,117)	(11,149)	$ (6,524)	$ (6,444)	$ (5,303)	(48,280)	(29,420)	$ (20,903)
Accounts Receivable, net	21,977				12,953				21,977	12,953
Accumulated Deficit	(203,515)				$ (156,589)				(203,515)	$ (156,589)
Without the adoption of Topic 606 | ASU 2014-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue									145,320
Sales, General & Administrative Expense									132,603
Net Loss									(51,274)
Accounts Receivable, net	17,629								17,629
Accumulated Deficit	(207,863)								(207,863)
Impact | ASU 2014-09
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Revenue									1,973
Sales, General & Administrative Expense									(1,021)
Net Loss									2,994
Accounts Receivable, net	4,348								4,348
Accumulated Deficit	$ 4,348								$ 4,348			$ 1,400

Cash Equivalents and Investme_3

Cash Equivalents and Investments - Schedule of Fair Value of Securities, not Including Cash (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 73,427	$ 102,325
Gross Unrealized Gains	3
Gross Unrealized Losses	(19)	(65)
Estimated Fair Value	73,411	102,260
Cash equivalents	15,091	5,574
Short-term investments	58,320	93,692
Long-term investments		2,994
Total	73,411	102,260
Money Market Funds
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	10,606	5,574
Estimated Fair Value	10,606	5,574
U.S. Government Securities
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,976	24,969
Gross Unrealized Losses	(1)	(29)
Estimated Fair Value	9,975	24,940
Corporate Notes
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	16,514	24,539
Gross Unrealized Gains	3
Gross Unrealized Losses	(18)	(36)
Estimated Fair Value	16,499	24,503
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	36,331	47,243
Estimated Fair Value	$ 36,331	$ 47,243

Cash Equivalents and Investme_4

Cash Equivalents and Investments - Additional Information (Details)	12 Months Ended
	Dec. 31, 2018Investment
Cash Equivalents And Investments [Abstract]
Available-for-sale securities, weighted average days to maturity	62 days
Number of unrealized loss position in investments	8

Cash Equivalents and Investme_5

Cash Equivalents and Investments - Schedule of Fair Value of Short-term and Long-term Marketable Securities Classified by Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Marketable Securities [Abstract]
Due within one year	$ 73,411	$ 99,266
Due after one year through three years		2,994
Total available-for-sale marketable debt securities	$ 73,411	$ 102,260

Cash Equivalents and Investme_6

Cash Equivalents and Investments - Schedule of Available-for-Sale Securities Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value	$ 17,498	$ 57,309
Available-for-sale securities, unrealized loss position, Less than 12 months, Unrealized Loss	(11)	(65)
Available-for-sale securities, unrealized loss position, Greater than 12 months, Fair Value	2,993
Available-for-sale securities, unrealized loss position, Greater than 12 months, Unrealized Loss	(8)
U.S. Government Securities
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value	5,977	24,940
Available-for-sale securities, unrealized loss position, Less than 12 months, Unrealized Loss	(1)	(29)
Corporate Notes
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value	11,521	24,503
Available-for-sale securities, unrealized loss position, Less than 12 months, Unrealized Loss	(10)	(36)
Available-for-sale securities, unrealized loss position, Greater than 12 months, Fair Value	2,993
Available-for-sale securities, unrealized loss position, Greater than 12 months, Unrealized Loss	$ (8)
Commercial Paper
Schedule Of Available For Sale Securities [Line Items]
Available-for-sale securities, unrealized loss position, Less than 12 months, Fair Value		$ 7,866

Fair Value Measurements - Addit

Fair Value Measurements - Additional Information (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Carrying amount
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Outstanding interest-bearing obligations	$ 34.9	$ 34
Estimated fair value | Level 2
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Outstanding interest-bearing obligations	$ 34.9	$ 38.6

Fair Value Measurements - Sched

Fair Value Measurements - Schedule of Fair Value of Company's Financial Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Assets
Total financial assets	$ 73,411	$ 102,260
Money Market Funds
Assets
Total financial assets	10,606	5,574
U.S. Government Securities
Assets
Total financial assets	9,975	24,940
Corporate Notes
Assets
Total financial assets	16,499	24,503
Commercial Paper
Assets
Total financial assets	36,331	47,243
Level 1
Assets
Total financial assets	10,606	5,574
Level 1 | Money Market Funds
Assets
Total financial assets	10,606	5,574
Level 2
Assets
Total financial assets	62,805	96,686
Level 2 | U.S. Government Securities
Assets
Total financial assets	9,975	24,940
Level 2 | Corporate Notes
Assets
Total financial assets	16,499	24,503
Level 2 | Commercial Paper
Assets
Total financial assets	$ 36,331	$ 47,243

Balance Sheet Components - Comp

Balance Sheet Components - Components of Inventory and Printed Circuit Board Assemblies ("PCBAs") (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Inventory [Line Items]
Total inventory and printed circuit board assemblies	$ 4,593	$ 4,933
Inventory	2,062	1,683
Other assets	3,208	3,465
Printed Circuit Board Assemblies
Inventory [Line Items]
Other assets	2,531	3,250
Raw Materials
Inventory [Line Items]
Total inventory and printed circuit board assemblies	1,028	1,103
Finished Goods
Inventory [Line Items]
Total inventory and printed circuit board assemblies	$ 3,565	$ 3,830

Balance Sheet Components - Co_2

Balance Sheet Components - Components of Property and Equipment, Net (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 14,388	$ 9,286
Less: accumulated depreciation and amortization	(5,230)	(3,065)
Total property and equipment, net	9,158	6,221
Laboratory and Manufacturing Equipment
Property Plant And Equipment [Line Items]
Total property and equipment, gross	2,750	1,994
Computer Equipment and Software
Property Plant And Equipment [Line Items]
Total property and equipment, gross	1,062	1,015
Furniture and Fixtures
Property Plant And Equipment [Line Items]
Total property and equipment, gross	925	953
Leasehold Improvements
Property Plant And Equipment [Line Items]
Total property and equipment, gross	726	726
Internal-Use Software
Property Plant And Equipment [Line Items]
Total property and equipment, gross	$ 8,925	$ 4,598

Balance Sheet Components - Addi

Balance Sheet Components - Additional Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property Plant And Equipment [Abstract]
Depreciation and amortization expense	$ 2,269	$ 1,597	$ 568

Balance Sheet Components - Co_3

Balance Sheet Components - Components of Accrued Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Payables And Accruals [Abstract]
Accrued vacation	$ 2,825	$ 2,081
Accrued payroll and related expenses	18,188	9,361
Accrued professional services fees	1,243	1,041
Claims payable	2,374	1,375
Other	1,940	1,786
Total accrued liabilities	$ 26,570	$ 15,644

Commitments and Contingencies -

Commitments and Contingencies - Schedule of Future Minimum Lease Payments Under Non-cancelable Operating Leases (Details) $ in Thousands	Dec. 31, 2018USD ($)
Operating Leases Future Minimum Payments Due [Abstract]
2,019	$ 8,135
2,020	10,669
2,021	10,828
2,022	11,150
2,023	11,483
Thereafter	98,209
Total	$ 150,474

Commitments and Contingencies_2

Commitments and Contingencies - Additional Information (Details)	Oct. 04, 2018USD ($)ft²	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)
Operating Leased Assets [Line Items]
Rent Expense		$ 5,500,000	$ 5,100,000	$ 3,100,000
Office Lease | San Francisco
Operating Leased Assets [Line Items]
Lease rentable area | ft²	117,560
Lease expected commencement date	Apr. 1, 2019
Lease term	12 years
Lease expiration date	Aug. 31, 2031
Renewal term of lease	5 years
Annual rental payments	$ 10,000,000
Percentage of increase to rental payments, each year	3.00%
Office Lease | San Francisco | Standby Letters of Credit
Operating Leased Assets [Line Items]
Proceeds from line of credit	$ 6,900,000
Office Lease | San Francisco | Maximum
Operating Leased Assets [Line Items]
Tenant improvement allowance	$ 2,400,000

Debt - Additional Information (

Debt - Additional Information (Details)	Oct. 23, 2018USD ($)	Oct. 31, 2018USD ($)	May 31, 2018USD ($)	Dec. 31, 2015USD ($)Tranche	Jun. 30, 2015	Nov. 30, 2012USD ($)shares	Dec. 31, 2018USD ($)	Dec. 31, 2017USD ($)	Aug. 31, 2016USD ($)
Debt Instrument [Line Items]
Repayment of debt							$ 31,500,000
Loss in connection with early extinguishment of debt							3,029,000
Proceeds from long-term debt, net of issuance costs							$ 35,000,000
Pharmakon Loan Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 55,000,000
Number of tranches for loans | Tranche				2
Loss in connection with early extinguishment of debt	$ 3,000,000
Prepayment premium fee	1,000,000
Additional consideration for prepayment of debt	1,500,000
Debt balance								$ 32,500,000
Repayment of debt		$ 35,800,000
Pharmakon Loan Agreement | Silicon Valley Bank
Debt Instrument [Line Items]
Repayment of debt				$ 4,900,000
Pharmakon Loan Agreement | Tranche A Loans
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 30,000,000
Debt instrument, fixed interest rate				9.50%
Debt instrument, periodic payment							quarterly
Debt instrument, interest due and payable				50.00%
Repayment of debt	30,000,000
Payment of accrued interest on debt	$ 3,300,000
Pharmakon Loan Agreement | Tranche B Loans
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 25,000,000
SVB Loan Agreement
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 15,000,000
Credit facility expiration date				Dec. 4, 2018
Borrowing capacity description							The Company may borrow up to 80% of its eligible accounts receivable, up to the maximum of $15.0 million
SVB Loan Agreement | Maximum
Debt Instrument [Line Items]
Percentage of eligible accounts receivable for borrowings				80.00%
SVB Loan Agreement | Prime Rate
Debt Instrument [Line Items]
Debt instrument interest rate spread				0.25%
SVB Loan Agreement | Standby Letters of Credit
Debt Instrument [Line Items]
Line of credit									$ 3,100,000
Third Amended and Restated SVB Loan Agreement
Debt Instrument [Line Items]
Debt balance							$ 40,611,000
Third Amended and Restated SVB Loan Agreement | Standby Letters of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity		11,000,000
Proceeds from line of credit		6,900,000
Third Amended and Restated SVB Loan Agreement | SVB Term Loan
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 35,000,000
Last interest only payment date		Oct. 31, 2020
Number of monthly payments of principal and interest		36 months
Debt instrument, interest rate terms							Interest charged on the SVB Term Loan will be the greater of (a) a floating rate based on the “Prime Rate” published by The Wall Street Journal minus 0.75%, or (b) 4.25%
Third Amended and Restated SVB Loan Agreement | SVB Term Loan | Minimum
Debt Instrument [Line Items]
Debt instrument, interest rate		4.25%
Third Amended and Restated SVB Loan Agreement | SVB Term Loan | Prime Rate
Debt Instrument [Line Items]
Basis spread deduction on variable rate		0.75%
Third Amended and Restated SVB Loan Agreement | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity		$ 25,000,000
Line of credit							$ 0
Debt instrument, interest rate terms							Any principal amount outstanding under the Third Amended and Restated SVB Loan Agreement revolving credit line shall bear interest at an amount that is the greater of (a) a floating rate per annum equal to the rate published by The Wall Street Journal as the “Prime Rate” or (b) 5.00%.
Third Amended and Restated SVB Loan Agreement | Revolving Credit Facility | Maximum
Debt Instrument [Line Items]
Percentage of eligible accounts receivable for borrowings		75.00%
Credit facility borrowing capacity							$ 1,700,000
Third Amended and Restated SVB Loan Agreement | Revolving Credit Facility | Minimum
Debt Instrument [Line Items]
Debt instrument, interest rate		5.00%
CHCF Note
Debt Instrument [Line Items]
Repayment of debt			$ 1,500,000
Payment of accrued interest on debt			$ 200,000
Debt balance								$ 1,500,000
Proceeds from long-term debt, net of issuance costs						$ 1,500,000
Debt instrument accrues simple interest rate						2.00%
Debt instrument maturity date					May 31, 2018	Nov. 30, 2016
CHCF Note | Series D Convertible Preferred Stock
Debt Instrument [Line Items]
Warrants issued to purchase shares | shares						22,807

Debt - Schedule of Future Minim

Debt - Schedule of Future Minimum Payments (Details) - Third Amended and Restated SVB Loan Agreement $ in Thousands	Dec. 31, 2018USD ($)
Debt Instrument [Line Items]
2,019	$ 1,662
2,020	3,603
2,021	12,983
2,022	12,429
2,023	9,934
Total	40,611
Less: Amount representing interest	(5,611)
Less: Amount representing debt issuance costs	(101)
Total carrying value	$ 34,899

Income Taxes - Additional Infor

Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
U.S. statutory tax rate	21.00%
Provision for income taxes	$ 44,000	$ 0	$ 0
Benefit recognized for net operating loss carryforwards	0
Benefit recognized for other deferred tax assets	0
Increase (decrease) in valuation allowance	21,700,000	(600,000)
Net operating loss carryforwards, federal	206,700,000
Net operating loss carryforwards, state	107,400,000
Tax credit carryforward	3,700,000
Tax credit carryforwards available to reduce future taxable income	$ 2,400,000
Tax credit carryforwards expiration beginning year	2,028
Unrecognized tax benefit	$ 1,500,000	900,000	$ 600,000
Accrual for interest or penalties related to unrecognized tax benefits	$ 0	$ 0
Federal
Income Tax Disclosure [Line Items]
Net operating loss carry forwards expiration year	2,027
State
Income Tax Disclosure [Line Items]
Net operating loss carry forwards expiration year	2,019

Income Taxes - Components of Pr

Income Taxes - Components of Provision for Income Taxes (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Current:
Foreign	$ 80,000
Total current tax expense	80,000
Deferred:
Foreign	(36,000)
Total deferred tax benefit	(36,000)
Provision for income taxes	$ 44,000	$ 0	$ 0

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of Tax Expense Computed at Statutory Federal Rate and Company's Tax Expense (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Expense Benefit Continuing Operations Income Tax Reconciliation [Abstract]
Tax at statutory federal rate	$ (10,143,000)	$ (10,034,000)	$ (7,107,000)
Stock-based compensation	(8,562,000)	(7,634,000)	255,000
Meals and Entertainment	309,000	198,000	134,000
Other	149,000	(145,000)	782,000
Tax credits	(1,015,000)	(644,000)	(139,000)
2017 Tax Act	44,000	21,928,000
Change in valuation allowance	19,262,000	(3,669,000)	6,075,000
Provision for income taxes	$ 44,000	$ 0	$ 0

Income Taxes - Significant Comp

Income Taxes - Significant Components of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred tax assets:
Net operating loss carryforwards	$ 50,334	$ 35,338
Tax credit carryforwards	4,287	2,642
Allowances and other	11,694	6,682
Depreciation and amortization	(388)	(414)
Total deferred tax assets	65,927	44,248
Valuation allowance	(65,886)	(44,248)	$ (44,861)	$ (38,565)
Net deferred tax assets	$ 41	$ 0

Income Taxes - Summary of Valua

Income Taxes - Summary of Valuation Allowance for Deferred Tax Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Valuation Allowance [Abstract]
Balance at beginning of year	$ 44,248	$ 44,861	$ 38,565
Additions	21,638		6,296
Deductions		613
Balance at end of year	$ 65,886	$ 44,248	$ 44,861

Income Taxes - Reconciliation_2

Income Taxes - Reconciliation of Unrecognized Tax Benefit (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation Of Unrecognized Tax Benefits Excluding Amounts Pertaining To Examined Tax Returns Roll Forward
Balance at beginning of year	$ 943	$ 616	$ 570
Additions for tax positions taken in current year	441	328	75
Increases in balance related to prior year tax positions	75
Decreases in balances related to prior year tax position		(1)	(29)
Balance at end of year	$ 1,459	$ 943	$ 616

Stockholders' Equity - Addition

Stockholders' Equity - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Equity [Abstract]
Common stock, shares authorized	100,000,000	100,000,000
Common stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, par value	$ 0.001	$ 0.001
Dividends declared	$ 0

Stockholders' Equity - Schedule

Stockholders' Equity - Schedule Of Common Stock Shares Reserved For Future Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Total number of shares reserved for future issuance	8,253,899	7,725,133
Common stock warrants issued and outstanding
Class Of Stock [Line Items]
Total number of shares reserved for future issuance	4,857	4,857
Options Issued and Outstanding
Class Of Stock [Line Items]
Total number of shares reserved for future issuance	2,094,137	2,601,181
Unvested restricted stock units
Class Of Stock [Line Items]
Total number of shares reserved for future issuance	547,891	468,426
Shares Available for Grant Under Future Stock Plans
Class Of Stock [Line Items]
Total number of shares reserved for future issuance	5,607,014	4,650,669

Stock Incentive Plans - Additio

Stock Incentive Plans - Additional Information (Details) $ / shares in Units, $ in Millions	1 Months Ended	12 Months Ended
	Oct. 31, 2016shares	Dec. 31, 2018USD ($)Period$ / sharesshares	Dec. 31, 2017USD ($)$ / sharesshares	Dec. 31, 2016USD ($)$ / sharesshares
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Options granted during the period		366,928	465,271	361,385
Shares available for future issuance		8,253,899	7,725,133
Expected term		6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days
Expected dividend yield		0.00%	0.00%	0.00%
Weighted-average grant date fair value of options | $ / shares		$ 32.38	$ 18.69	$ 8.76
Stock option, intrinsic value | $		$ 52.4	$ 32.5	$ 0.7
Estimated grant date fair value of stock option vested | $		$ 5.3	$ 2.4	$ 1.8
2006 Equity Incentive Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option plan, description		Options under the Plan were granted for periods of up to ten years and at the fair value of the shares on the date of grant as determined by the board of directors.
Stock option vesting term		3 years
2006 Equity Incentive Plan | First Anniversary of Grant
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option exercisable rate, Percentage		25.00%
2006 Equity Incentive Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option, period for grant		10 years
2006 Equity Incentive Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of voting power of all classes of stock		10.00%
2006 Equity Incentive Plan | ISO | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting term		5 years
2006 Equity Incentive Plan | Other Options | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option vesting term		10 years
2016 Plan
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option plan, description		On the first day of each year, the 2016 Plan authorizes an annual increase of the least of 3,865,000 shares, 5% of outstanding shares on the last day of the immediately preceding fiscal year or an amount as determined by the Company's Board of Directors.
Effective date	Oct. 19, 2016
Options granted during the period	0
Shares available for future issuance	3,865,000	6,140,831
Percentage of outstanding shares increased annually under the plan	5.00%
2016 Plan | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock option, period for grant				10 years
2016 Plan | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Percentage of stock option exercise price of fair market value of common stock	100.00%
Employee Stock Purchase Program ("ESPP")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Effective date	Oct. 19, 2016
Shares available for future issuance	483,031	889,722
Percentage of outstanding shares increased annually under the plan	1.50%
Increase in shares available for future issuance	966,062
Percentage of payroll deductions of eligible compensation	15.00%
Offering period		12 months
Number of purchase period | Period		2
Purchase period		6 months
Percentage of employee purchase shares of fair market value	85.00%
Common stock issued to employees		276,058
Expected volatility, minimum		41.46%
Expected volatility, maximum		44.55%
Risk-free interest rate, minimum		2.19%
Risk-free interest rate, maximum		2.64%
Expected dividend yield		0.00%
Employee Stock Purchase Program ("ESPP") | Maximum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term		1 year
Employee Stock Purchase Program ("ESPP") | Minimum
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected term		6 months

Stock Incentive Plans - Summary

Stock Incentive Plans - Summary of Share-based Awards Available for Grant under 2016 Plan (Details) - 2016 Plan - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Available for Grant, Beginning balance	4,034,152	3,743,037	331,938
Shares Available for Grant, Additional awards authorized	1,168,865	1,106,966	3,865,000
Shares Available for Grant, Awards granted	(666,913)	(837,436)	(466,914)
Shares Available for Grant, Awards forfeited	124,478	21,585	13,013
Shares Available for Grant, Awards withheld for tax purposes	56,710
Shares Available for Grant, Ending balance	4,717,292	4,034,152	3,743,037

Stock Incentive Plans - Summa_2

Stock Incentive Plans - Summary of Stock Option Activity Under 2006 and 2016 Plans, Including Grants To Nonemployees (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Options Outstanding, Beginning balance	2,601,181	2,977,218	2,685,913
Options granted	366,928	465,271	361,385
Options exercised	(798,424)	(827,556)	(57,067)
Options forfeited	(75,548)	(13,752)	(13,013)
Options Outstanding, Ending balance	2,094,137	2,601,181	2,977,218	2,685,913
Options exercisable	1,278,367
Options vested and expected to vest	2,050,504
Options Outstanding, Weighted Average Exercise Price Per Share, Beginning Balance	$ 12.24	$ 6.16	$ 4.81
Options Outstanding, Weighted Average Exercise Price Per Share, Options granted	68.32	36.76	15.65
Options Outstanding, Weighted Average Exercise Price Per Share, Options exercised	7.19	4.11	2.33
Options Outstanding, Weighted Average Exercise Price Per Share, Options forfeited	34.30	14.43	6.92
Options Outstanding, Weighted Average Exercise Price Per Share, Ending Balance	23.20	$ 12.24	$ 6.16	$ 4.81
Options Outstanding, Weighted Average Exercise Price Per Share, Exercisable	10.27
Options Outstanding, Weighted Average Exercise Price Per Share, vested and expected to vest	$ 22.60
Options Outstanding, Weighted- Average Remaining Contractual Life (years)	7 years 7 days	7 years 2 months 1 day	6 years 11 months 4 days	7 years 7 months 17 days
Options exercisable, Weighted- Average Remaining Contractual Life (years)	6 years 2 months 4 days
Options vested and expected to vest, Weighted- Average Remaining Contractual Life (years)	6 years 11 months 26 days
Options Outstanding, Aggregate Intrinsic Value	$ 97,976	$ 113,958	$ 70,979	$ 11,589
Options exercisable, Aggregate Intrinsic Value	75,695
Options vested and expected to vest, Aggregate Intrinsic Value	$ 97,085

Stock Incentive Plans - Summa_3

Stock Incentive Plans - Summary of Non-vested Restricted Stock Units ("RSUs") (Details) - Restricted Stock Units ("RSUs") - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Shares Underlying RSUs, Non-vested, Beginning balance	468,426
Shares Underlying RSUs, Granted	299,985
Shares Underlying RSUs, Vested and released	(114,880)
Shares Underlying RSUs, Forfeited	(105,640)
Shares Underlying RSUs, Non-vested, Ending balance	547,891	468,426
Shares Underlying RSUs, Non-vested Outstanding, Weighted Average Grant Date Fair Value, Beginning Balance	$ 36.96
Shares Underlying RSUs, Granted, Weighted Average Grant Date Fair Value	71.54
Shares Underlying RSUs, Vested and released, Weighted Average Grant Date Fair Value	31.53
Shares Underlying RSUs, Forfeited, Weighted Average Grant Date Fair Value	38.81
Shares Underlying RSUs, Non-vested Outstanding, Weighted Average Grant Date Fair Value, Ending Balance	$ 56.62	$ 36.96
Shares Underlying RSUs, Non-vested Weighted Remaining Vesting Period (in years)	2 years 5 months 12 days	2 years 6 months 14 days
Shares Underlying RSUs, Non-vested, Aggregate Intrinsic Value	$ 38,067	$ 26,255

Stock-Based Compensation - Sche

Stock-Based Compensation - Schedule of Fair Value of Employee Stock Options Estimated Using Weighted Average Assumptions (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Expected term (in years)	6 years 1 month 6 days	6 years 1 month 6 days	6 years 1 month 6 days
Expected volatility	45.70%	51.90%	60.00%
Risk-free interest rate	2.75%	2.07%	1.42%
Dividend yield	0.00%	0.00%	0.00%

Stock-Based Compensation - Addi

Stock-Based Compensation - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Expected dividend yield	0.00%	0.00%	0.00%
Total unamortized compensation costs, net of estimated forfeitures related to unvested stock options	$ 14.1
Unamortized compensation costs related to unvested stock options, expected period of recognition	2 years 3 months 18 days
Restricted Stock Units ("RSUs")
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Unamortized compensation costs related to unvested stock options, expected period of recognition	2 years 6 months
Total unamortized compensation costs, net of estimated forfeitures related to restricted stock unit	$ 21.8
ESPP
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Total unamortized compensation costs, net of estimated forfeitures related to unvested stock options	$ 1.3
Unamortized compensation costs related to unvested stock options, expected period of recognition	10 months 24 days

Stock-Based Compensation - Summ

Stock-Based Compensation - Summary of Total Stock-Based Compensation Expense for Options, RSUs and ESPP Included in Consolidated Statements of Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 16,329	$ 10,123	$ 1,871
Cost of Revenue
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	193	163	17
Research and development
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	3,057	1,733	203
Selling, general and administrative
Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Line Items]
Total stock-based compensation expense	$ 13,079	$ 8,227	$ 1,651

Net Loss Per Common Share - Com

Net Loss Per Common Share - Computation of Basic and Diluted Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net loss	$ (14,713)	$ (10,244)	$ (12,206)	$ (11,117)	$ (11,149)	$ (6,524)	$ (6,444)	$ (5,303)	$ (48,280)	$ (29,420)	$ (20,903)
Denominator:
Weighted-average shares used to compute net loss per common share, basic and diluted									23,885,858	22,627,327	5,285,847
Net loss per common share, basic and diluted	$ (0.61)	$ (0.43)	$ (0.51)	$ (0.47)	$ (0.48)	$ (0.29)	$ (0.29)	$ (0.24)	$ (2.02)	$ (1.30)	$ (3.95)

Net Loss Per Common Share - Sch

Net Loss Per Common Share - Schedule of Anti-dilutive Securities Excluded from Diluted Net Loss per Common Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per common share	2,646,885	3,074,464	3,299,992
Options to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per common share	2,094,137	2,601,181	2,977,218
Restricted Stock Units ("RSUs")
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per common share	547,891	468,426	105,529
Warrants to Purchase Common Stock
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Anti-dilutive securities excluded from diluted net loss per common share	4,857	4,857	217,245

Selected Quarterly Financial _3

Selected Quarterly Financial Data (Unaudited) - Schedule of Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 43,155	$ 38,104	$ 35,469	$ 30,565	$ 28,183	$ 25,035	$ 23,854	$ 21,437	$ 147,293	$ 98,509	$ 64,072
Gross profit	32,626	28,155	25,979	21,954	20,476	18,115	17,110	15,100	108,714	70,801	43,189
Net loss	$ (14,713)	$ (10,244)	$ (12,206)	$ (11,117)	$ (11,149)	$ (6,524)	$ (6,444)	$ (5,303)	$ (48,280)	$ (29,420)	$ (20,903)
Net loss per common share, basic and diluted	$ (0.61)	$ (0.43)	$ (0.51)	$ (0.47)	$ (0.48)	$ (0.29)	$ (0.29)	$ (0.24)	$ (2.02)	$ (1.30)	$ (3.95)

Subsequent Events - Additional

Subsequent Events - Additional Information (Details) - Subsequent Event - Third Amended Lincolnshire Lease	Feb. 22, 2019USD ($)Right
Subsequent Event [Line Items]
Renewal term of sublease	2 years
Estimated annual rent	$ 700,000
Number of terms, right to extend sublease | Right	2
Maximum
Subsequent Event [Line Items]
Tenant improvement allowance	$ 60,000