Reinsurance - External Reinsurance on Income Statements (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Written premiums:
Assumed	$ 1,293.3	$ 1,273.5	$ 1,194.2
Ceded	(815.5)	(823.4)	(858.2)
Net written premiums	477.8	450.1	336.0
Earned premiums:
Direct	840.6	863.1	882.6
Assumed	1,291.9	1,270.5	1,074.1
Ceded	(819.9)	(832.9)	(861.7)
Losses and loss expenses incurred:
Assumed	944.1	864.2	774.4
Ceded	(560.3)	(570.9)	(523.6)
Net losses and loss expenses incurred	383.8	293.3	250.8
Prior to reinsurance transaction with State Auto Mutual
Written premiums:
Direct	835.4	854.1	880.6
Assumed	5.0	4.7	4.4
Ceded	(24.9)	(35.4)	(26.8)
Net written premiums	815.5	823.4	858.2
Earned premiums:
Direct	840.6	863.1	882.6
Assumed	4.9	4.5	4.4
Ceded	(25.6)	(34.7)	(25.3)
Net earned premiums	819.9	832.9	861.7
Losses and loss expenses incurred:
Direct	559.1	569.0	525.6
Assumed	3.9	3.4	2.9
Ceded	(4.4)	(2.9)	(8.0)
Net losses and loss expenses incurred	$ 558.6	$ 569.5	$ 520.5