Reinsurance - External Reinsurance on Balance Sheets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Losses and loss expenses payable:
Assumed	$ 1,252.5	$ 1,178.0
Ceded	(541.1)	(547.1)
Unearned premiums:
Assumed	605.4	611.7
Ceded	(417.5)	(390.8)
Unearned Premiums From Affiliates	187.9	220.9
Prior to reinsurance transaction with State Auto Mutual
Losses and loss expenses payable:
Direct	535.0	545.7
Assumed	9.2	5.0
Ceded	(3.1)	(3.6)
Net losses and loss expenses payable	541.1	547.1
Unearned premiums:
Direct	402.5	395.6
Assumed	21.4	1.3
Ceded	(6.4)	(6.1)
Unearned Premiums From Affiliates	$ 417.5	$ 390.8