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=123359005&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
-Paragraph 20
-URI http://asc.fasb.org/extlink&oid=123594938&loc=d3e13531-108611

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123594938&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

Name:	us-gaap_ConcentrationRiskPercentage1
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

Segment reporting - Narrative (Details)	3 Months Ended	12 Months Ended
Segment reporting - Narrative (Details)	Dec. 31, 2021 segment	Dec. 31, 2021 segment product	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Number of operating segments \| segment	2	2
Gross premiums written \| Credit concentration risk
Segment Reporting Information [Line Items]
% of gross premiums written		100.00%	100.00%	100.00%
Largest contract \| Gross premiums written \| Credit concentration risk
Segment Reporting Information [Line Items]
% of gross premiums written				14.40%
Reinsurance \| Credit & Bond \| Minimum
Segment Reporting Information [Line Items]
Coverage period		60 days
Reinsurance \| Credit & Bond \| Maximum
Segment Reporting Information [Line Items]
Coverage period		120 days
Insurance & Services \| Environmental
Segment Reporting Information [Line Items]
Number of core products \| product		4