Summary of Significant Accounting Policies - Intangible Assets - Additional Information (Details) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Business Acquisition [Line Items]
Amortization of intangible assets	$ 100,000	$ 100,000
Impairment of indefinite-lived intangible assets	0
Surety
Business Acquisition [Line Items]
Goodwill, Impairment Loss	0
Casualty
Business Acquisition [Line Items]
Goodwill, Impairment Loss	$ 0

The aggregate expense charged against earnings to allocate the cost of intangible assets (nonphysical assets not used in production) in a systematic and rational manner to the periods expected to benefit from such assets. As a noncash expense, this element is added back to net income when calculating cash provided by or used in operations using the indirect method.

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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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Amount of loss from the write-down of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

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Amount of impairment loss resulting from write-down of assets, excluding financial assets and goodwill, lacking physical substance and having a projected indefinite period of benefit to fair value.

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