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, 2016	Mar. 31, 2015
Premiums earned:
Assumed from external insurers and reinsurers	$ 1.4	$ 1.1
Assumed under Pooling Arrangement	319.9	315.3
Ceded to external insurers and reinsurers	(6.8)	(8.9)
Ceded under Pooling Arrangement	(206.5)	(209.9)
Net assumed premiums earned	108.0	97.6
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	1.1	0.8
Assumed under Pooling Arrangement	225.4	195.3
Ceded to external insurers and reinsurers	(1.9)	0.1
Ceded under Pooling Arrangement	(131.7)	(121.2)
Net assumed losses and loss expenses incurred	$ 92.9	$ 75.0