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Statutory Information - Narrative (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2020 USD ($) subsidiary	Dec. 31, 2019 USD ($)
Bermuda
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	[1]	$ 6,431,413	$ 5,492,968
Ireland
Statutory Accounting Practices [Line Items]
Number Of Subsidiaries | subsidiary		3
Required statutory capital and surplus	[1]	$ 701,161	542,703
United States
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	[1]	$ 1,644,324	$ 1,697,640
Mortgage | North Carolina or Wisconsin
Statutory Accounting Practices [Line Items]
Required contingency loss reserve as a percentage of net earned premiums		50.00%
Withdrawal restriction period		10 years
Withdrawal restriction period, with prior approval		35.00%
Maximum risk to capital ratio		2500.00%
United Guaranty Residential Insurance Company [Member] | North Carolina
Statutory Accounting Practices [Line Items]
Dividends available for payment in 2020, total		$ 143,100
Arch Re US | United States
Statutory Accounting Practices [Line Items]
Dividends available for payment in 2020, total		$ 147,600
Arch Re Bermuda | Bermuda
Statutory Accounting Practices [Line Items]
Dividends, percentage permitted		25.00%
Dividend required notice period		seven days
Dividends available for payment in 2021 without prior regulatory approval		$ 3,800,000
[1] Such amounts include ownership interests in affiliated insurance and reinsurance subsidiaries.

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- Definition

Maximum Risk To Capital Ratio

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Number Of Subsidiaries

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- Definition

Statutory Accounting Contingency Loss Reserve Percentage Of Net Earned Premiums

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- Definition

Statutory Accounting Contingency Loss Reserve Percentage Of Net Earned Premiums With Prior Approval

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- Definition

The percentage of total statutory capital and surplus (as shown on its previous financial year's statutory balance sheet) which may be paid out as dividends without the requirement to file an affadavit with the regulatory body stating that the Company will continue to meet required margins.

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Statutory Capital And Surplus Notice Period Required For Dividends

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Withdrawal Restriction Period

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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- Definition

Amount of dividends available for payment after computation of statutory restrictions on capital surplus.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(e))
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Amount of dividends that can be declared or paid without approval of regulatory agency.

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Amount of statutory capital required to be maintained as of the balance sheet date under prescribed or permitted statutory accounting practices.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 505
-Section 50
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