Summary of Significant Accounting Policies - Narrative (Details)	3 Months Ended	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2019 USD ($) embedded_leases_with_suppliers	Sep. 30, 2018 USD ($)	Dec. 31, 2018	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)	Oct. 04, 2018 ft²
Summary Of Significant Accounting Policies [Line Items]
Total revenue recognized that was included in contract liability		$ 1,200,000	$ 1,200,000
Number of embedded leases with suppliers | embedded_leases_with_suppliers		0
Operating lease right-of-use assets		$ 91,935,000			$ 94,243,000	$ 8,962,000
Operating lease liabilities		$ 95,135,000
Accounting Standards Codification 606 cumulative effect adjustment upon adoption							$ 0
Topic 842
Summary Of Significant Accounting Policies [Line Items]
Operating lease right-of-use assets							10,200,000
Operating lease liabilities							$ 10,000,000.0
Office lease | San Francisco
Summary Of Significant Accounting Policies [Line Items]
Lease rentable area (in square feet) | ft²								117,560
Zio XT service
Summary Of Significant Accounting Policies [Line Items]
Period from device applied to the time the report is posted (in days)		20 days
Maximum | Zio XT service
Summary Of Significant Accounting Policies [Line Items]
Equipment wear period (in days)		14 days
Revenue | Customer Concentration Risk | Federal Government Agencies
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk (in percentage)	38.00%	36.00%	37.00%	33.00%
Accounts Receivable | Accounts Receivable Concentration Risk | Federal Government Agencies
Summary Of Significant Accounting Policies [Line Items]
Concentration of credit risk (in percentage)		21.00%		17.00%