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, 2018	Mar. 31, 2017	Mar. 01, 2018
Insurance [Abstract]
Unearned Premiums Ceded			$ 9.7
Premiums earned:
Assumed from external insurers and reinsurers	$ 10.4	$ 1.1
Assumed under Pooling Arrangement	314.9	317.3
Ceded to external insurers and reinsurers	(5.4)	(6.1)
Ceded under Pooling Arrangement	(216.9)	(202.5)
Net assumed premiums earned	103.0	109.8
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	10.9	1.6
Assumed under Pooling Arrangement	202.7	234.4
Ceded to external insurers and reinsurers	(1.3)	(2.0)
Ceded under Pooling Arrangement	(156.5)	(148.2)
Net assumed losses and loss expenses incurred	$ 55.8	$ 85.8