Transactions with Affiliates (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Transactions with Affiliates
Premiums earned, Assumed from Old Republic	$ 96.2	$ 89.4	$ 75.1
Premiums earned, Ceded to Old Republic	2,496.7	2,310.5	2,039.0
American Business & Mercantile Insurance Mutual [Member]
Transactions with Affiliates
Premiums earned, Assumed from Old Republic	0.7	0.3	1.5
Premiums earned, Ceded to Old Republic	0.0	0.1	0.2
Commission and fees, Assumed from Old Republic	0.1	0.1	0.4
Commissions and fees, Ceded to Old Republic	0.0	0.0	0.0
Losses and loss expenses, Assumed from Old Republic	2.9	0.9	1.1
Losses and loss expenses, Ceded to Old Republic	0.0	0.1	0.5
Loss and loss expense reserves, Assumed from Old Republic	4.3	7.1	8.5
Loss and loss expense reserves, Ceded to Old Republic	1.4	2.7	3.1
Unearned premiums, Assumed from Old Republic	0.0	0.0	0.0
Unearned premiums, Ceded to Old Republic	0.0	0.0	$ 0.0
Mutual's statutory capital included surplus notes due to Old Republic	10.5	10.5
Total statutory capital	$ 62.4	$ 58.8