Investments - Additional Information (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($) security	Sep. 30, 2020 security
Investments
Number of debt securities held	9	7	7
Pre-tax net realized capital gains or losses \| $	$ 0.0	$ 0.0
Number of debt securities held, unrealized loss position for 12 months or more	4	6	0

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=121582814&loc=SL82922888-210455

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant