Acquisition (Details) - Protective Insurance - USD ($)		3 Months Ended	4 Months Ended
	Jun. 01, 2021	Sep. 30, 2021	Sep. 30, 2021
Business Combination, Separately Recognized Transactions [Line Items]
Price per common share	$ 23.30
Aggregate purchase amount	$ 338,000,000
Percent of company wide net premiums written		1.00%
Tax basis goodwill	0
Assets acquired	1,700,000,000
Investment securities acquired	1,100,000,000
Cash and cash equivalents, restricted cash and restricted cash equivalents acquired	24,300,000
Reinsurance recoverables acquired	452,400,000
Liabilities assumed in acquisition	1,400,000,000
Unearned premiums	66,900,000
Loss and loss adjustment expense reserves	1,100,000,000
Repayments of Lines of Credit	20,000,000
Revenue included in consolidated results		$ 144,300,000	$ 190,800,000
Net income included in consolidated results		$ 11,100,000	$ 13,400,000
Trade name
Business Combination, Separately Recognized Transactions [Line Items]
Intangible assets acquired, related to trade name	$ 3,600,000
Finite-Lived Intangible Asset, Useful Life	2 years