Transactions with Affiliates - Narrative (Details) $ in Millions		1 Months Ended		12 Months Ended
		May 31, 2009 USD ($) internationalFund	May 31, 2003 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Document Period End Date				Dec. 31, 2018
Number of days intercompany balances settled after quarter end				60 days
Company's premium share balances due				$ 347.8	$ 323.1
Mandatorily redeemable preferred capital securities issued to third party			$ 15.0
Common securities issued to state auto financial			0.5	$ 15.4	$ 10.2	$ 2.2
Proceeds from the issuance of Trust Securities			$ 15.5
Description of variable rate basis			three-month LIBOR rate plus 4.20%
Subordinated Borrowing, Due Date			Dec. 31, 2033
Interest rate (percent)				5.28%	5.28%
Number of credit agreements (agreements) | internationalFund	[1]	2
Credit agreement loan amount		$ 70.0		$ 70.0	$ 70.0
Fixed annual interest rate (percent)		7.00%
Interest income earned				4.9	4.9	4.9
Revenue from Contract with Customer, Excluding Assessed Tax				$ 2.0	$ 2.0	$ 1.9
Affiliate Subordinated Debentures due 2033
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Interest rate (percent)				6.94%	5.68%
3 month LIBOR | Affiliate Subordinated Debentures due 2033
Impact Of Pooling Arrangement Amendment On Balance Sheet [Line Items]
Interest rate of Trust Securities payable quarterly (percent)			4.20%

[1] In May 2009, the Company entered into 2 separate credit agreements with State Auto Mutual pursuant to which it loaned State Auto Mutual a total of $70.0 million.