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		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Premiums earned:
Assumed from external insurers and reinsurers	$ 1.3	$ 1.0	$ 2.4	$ 2.4
Assumed under Pooling Arrangement	320.1	322.4	637.4	642.3
Ceded to external insurers and reinsurers	(6.1)	(6.5)	(12.2)	(13.3)
Ceded under Pooling Arrangement	(202.0)	(204.8)	(404.5)	(411.3)
Net assumed premiums earned	113.3	112.1	223.1	220.1
Losses and loss expenses incurred:
Assumed from external insurers and reinsurers	0.9	0.8	2.5	1.9
Assumed under Pooling Arrangement	230.8	263.3	465.2	488.7
Ceded to external insurers and reinsurers	1.0	(0.5)	(1.0)	(2.4)
Ceded under Pooling Arrangement	(147.4)	(176.5)	(295.6)	(308.2)
Net assumed losses and loss expenses incurred	$ 85.3	$ 87.1	$ 171.1	$ 180.0