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		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Premiums earned:
Assumed from external insurers and reinsurers	$ 2.3	$ 1.2	$ 4.7	$ 3.6
Assumed under Pooling Arrangement	319.0	325.8	956.4	968.1
Ceded to external insurers and reinsurers	(4.9)	(6.0)	(17.1)	(19.3)
Ceded under Pooling Arrangement	(203.7)	(204.4)	(608.2)	(615.7)
Net assumed premiums earned	112.7	116.6	335.8	336.7
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	2.2	0.9	4.7	2.8
Assumed under Pooling Arrangement	254.0	232.8	719.2	721.5
Ceded to external insurers and reinsurers	(2.7)	0.3	(3.7)	(2.1)
Ceded under Pooling Arrangement	(125.1)	(131.9)	(420.7)	(440.1)
Net assumed losses and loss expenses incurred	$ 128.4	$ 102.1	$ 299.5	$ 282.1