Reinsurance - External Reinsurance on Income Statements (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Written premiums:
Assumed	$ 1,210.3	$ 1,270.3	$ 1,292.5
Ceded	(967.5)	(843.8)	(815.5)
Net written premiums	242.8	426.5	477.0
Earned premiums:
Direct	881.3	828.7	840.6
Assumed	1,238.0	1,276.1	1,291.1
Ceded	(919.7)	(817.1)	(819.9)
Losses and loss expenses incurred:
Assumed	798.1	919.8	944.1
Ceded	(617.6)	(540.4)	(560.3)
Net losses and loss expenses incurred	180.5	379.4	383.8
Prior to reinsurance transaction with State Auto Mutual
Written premiums:
Direct	915.1	835.6	835.4
Assumed	75.1	31.1	5.0
Ceded	(22.7)	(22.9)	(24.9)
Net written premiums	967.5	843.8	815.5
Earned premiums:
Direct	881.3	828.7	840.6
Assumed	61.0	11.0	4.9
Ceded	(22.6)	(22.6)	(25.6)
Net earned premiums	919.7	817.1	819.9
Losses and loss expenses incurred:
Direct	572.5	530.9	559.1
Assumed	48.4	10.4	3.9
Ceded	(4.9)	(2.5)	(4.4)
Net losses and loss expenses incurred	$ 616.0	$ 538.8	$ 558.6