Fair Value of Financial Instruments - Narrative (Details) $ in Millions		1 Months Ended
		May 31, 2009 USD ($) internationalFund	Dec. 31, 2018 USD ($) internationalFund	Dec. 31, 2017 USD ($)
Fair Value Measurements Disclosure [Line Items]
Other invested assets fair value			$ 38.5	$ 45.2
Other investments and securities, at cost			5.6	5.6
Number of credit agreements (agreements) | internationalFund	[1]	2
Notes receivable from affiliate		$ 70.0	70.0	$ 70.0
Fair Value, Inputs, Level 3
Fair Value Measurements Disclosure [Line Items]
Federal home loan bank stock			5.1
Other investments and securities, at cost			$ 0.5
International funds | Fair Value, Inputs, Level 3
Fair Value Measurements Disclosure [Line Items]
Number of available for sale securities (securities) | internationalFund			1

[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.