Reinsurance - External Reinsurance on Balance Sheets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Mar. 01, 2018	Dec. 31, 2017
Losses and loss expenses payable:
Assumed	$ 1,141.3		$ 1,252.5
Ceded	(547.7)		(541.1)
Unearned premiums:
Assumed	577.6		605.4
Ceded	(465.2)	$ 9.7	(417.5)
Unearned Premiums From Affiliates	112.4		187.9
Prior to reinsurance transaction with State Auto Mutual
Losses and loss expenses payable:
Direct	530.9		535.0
Assumed	22.3		9.2
Ceded	(5.5)		(3.1)
Net losses and loss expenses payable	547.7		541.1
Unearned premiums:
Direct	436.2		402.5
Assumed	35.5		21.4
Ceded	(6.6)		(6.4)
Unearned Premiums From Affiliates	$ 465.1		$ 417.5