Allowance for Expected Credit Losses - Ceded credit risk (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2020	Dec. 31, 2019
Ceded Credit Risk [Line Items]
Reinsurance recoverable on unpaid and paid losses and LAE	$ 4,640	$ 4,350
Reinsurance recoverable \| Credit concentration risk \| AM Best A minus Or Better Rating
Ceded Credit Risk [Line Items]
Concentration risk percentage	63.20%	61.20%
Reinsurance recoverable \| Credit concentration risk \| Not Rated
Ceded Credit Risk [Line Items]
Concentration risk percentage	36.60%	38.80%
Reinsurance recoverable \| Credit concentration risk \| Not Rated \| Reinsurance trusts or letters of credit
Ceded Credit Risk [Line Items]
Concentration risk percentage	90.00%
Stockholders' equity \| Reinsurer concentration risk
Ceded Credit Risk [Line Items]
Concentration risk percentage	1.80%	1.70%

For an entity that discloses a concentration risk in relation to quantitative amount, which serves as the "benchmark" (or denominator) in the equation, this concept represents the concentration percentage derived from the division.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 42
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e9054-108599

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6351-108592

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
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-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
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-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13537-108611

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 275
-SubTopic 10
-Section 50
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=99393423&loc=d3e6404-108592

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration