Reinsurance - External Reinsurance on Income Statements (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Written premiums:
Assumed	$ 1,269.3	$ 1,293.3	$ 1,273.5
Ceded	(843.8)	(815.5)	(823.4)
Net written premiums	425.5	477.8	450.1
Earned premiums:
Direct	828.7	840.6	863.1
Assumed	1,275.1	1,291.9	1,270.5
Ceded	(817.1)	(819.9)	(832.9)
Losses and loss expenses incurred:
Assumed	920.0	944.1	864.2
Ceded	(540.4)	(560.3)	(570.9)
Net losses and loss expenses incurred	379.6	383.8	293.3
Prior to reinsurance transaction with State Auto Mutual
Written premiums:
Direct	835.6	835.4	854.1
Assumed	31.1	5.0	4.7
Ceded	(22.9)	(24.9)	(35.4)
Net written premiums	843.8	815.5	823.4
Earned premiums:
Direct	828.7	840.6	863.1
Assumed	11.0	4.9	4.5
Ceded	(22.6)	(25.6)	(34.7)
Net earned premiums	817.1	819.9	832.9
Losses and loss expenses incurred:
Direct	530.9	559.1	569.0
Assumed	10.4	3.9	3.4
Ceded	(2.5)	(4.4)	(2.9)
Net losses and loss expenses incurred	$ 538.8	$ 558.6	$ 569.5