Summary of Significant Accounting Policies - Contract acquisition costs (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2020
Summary of Significant Accounting Policies
Deferred asset	$ 3.4			$ 3.4			$ 2.3
Amortized commissions	$ 1.3	$ 0.1	$ 0.1	$ 1.6	$ 0.3	$ 0.1

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68051541&loc=SL49131252-203054

Name:	us-gaap_CapitalizedContractCostNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant