Reinsurance - Summary of External Reinsurance Transactions, as well as reinsurance transactions with State Auto Mutual Under the Pooling Arrangement (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Insurance [Abstract]
Profit Commission			$ 2.9
Premiums earned:
Assumed from external insurers and reinsurers	1.2	1.2	3.3	2.7
Assumed under Pooling Arrangement	270.2	266.0	801.0	790.8
Ceded to external insurers and reinsurers	(6.5)	(5.4)	(18.5)	(17.6)
Ceded under Pooling Arrangement	(215.2)	(215.2)	(647.4)	(639.0)
Net assumed premiums earned	49.7	46.6	138.4	136.9
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	0.8	0.9	2.3	1.3
Assumed under Pooling Arrangement	185.2	179.5	548.9	540.1
Ceded to external insurers and reinsurers	(2.7)	(0.8)	(6.4)	(4.9)
Ceded under Pooling Arrangement	(131.6)	(137.9)	(417.3)	(434.0)
Net assumed losses and loss expenses incurred	$ 51.7	$ 41.7	$ 127.5	$ 102.5