Reinsurance - Summary of External Reinsurance Transactions, as well as reinsurance transactions with State Auto Mutual Under the Pooling Arrangement (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Premiums earned:
Assumed from external insurers and reinsurers	$ 20.6	$ 18.2
Assumed under Pooling Arrangement	330.5	302.7
Ceded to external insurers and reinsurers	(10.6)	(5.9)
Ceded under Pooling Arrangement	(272.2)	(243.0)
Net assumed premiums earned	68.3	72.0
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	11.7	14.2
Assumed under Pooling Arrangement	239.4	200.2
Ceded to external insurers and reinsurers	(33.5)	(0.6)
Ceded under Pooling Arrangement	(228.5)	(143.8)
Net assumed losses and loss expenses incurred	$ (10.9)	$ 70.0