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		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Premiums earned:
Assumed from external insurers and reinsurers	$ 1.1	$ 1.1	$ 2.1	$ 1.5
Assumed under Pooling Arrangement	268.3	263.5	530.8	524.8
Ceded to external insurers and reinsurers	(6.0)	(6.7)	(12.0)	(12.2)
Ceded under Pooling Arrangement	(216.8)	(213.1)	(432.2)	(423.8)
Net assumed premiums earned	46.6	44.8	88.7	90.3
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	0.8	0.6	1.5	0.4
Assumed under Pooling Arrangement (a)	190.4	186.1	362.2	359.1
Ceded to external insurers and reinsurers	(1.2)	(3.1)	(3.7)	(4.1)
Ceded under Pooling Arrangement (a)	(143.5)	(164.4)	(285.7)	(296.2)
Net assumed losses and loss expenses incurred	46.5	19.2	74.3	59.2
Gaap Expense Adjustments	$ 0.8	$ 0.8	$ 1.6	$ 1.5