Reinsurance - External Reinsurance on Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Losses and loss expenses payable:
Assumed	$ 1,178.0	$ 1,047.1
Ceded	(547.1)	(514.7)
Unearned premiums:
Assumed	611.7	609.5
Ceded	(390.8)	(395.3)
Unearned Premiums From Affiliates	220.9	214.2
Prior to reinsurance transaction with State Auto Mutual
Losses and loss expenses payable:
Direct	545.7	515.6
Assumed	5.0	5.0
Ceded	(3.6)	(5.9)
Net losses and loss expenses payable	547.1	514.7
Unearned premiums:
Direct	395.6	400.9
Assumed	1.3	1.2
Ceded	(6.1)	(6.8)
Unearned Premiums From Affiliates	$ 390.8	$ 395.3