Stockholders' Equity - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity, Class of Treasury Stock [Line Items]
Statutory Accounting Policies, Statutory Percent Available for Dividend Payments without Regulatory Approval	10.00%
Amount of dividends available for distributions without prior approval	$ 84.4
Statutory surplus in excess of authorized level, no remedial action required (percent)	200.00%
Statutory surplus in excess of authorized level, level maintained by company and subsidiaries (percent)	400.00%
State Auto Financial
Equity, Class of Treasury Stock [Line Items]
Dividends received from insurance subsidiaries	$ 10.0	$ 15.0	$ 20.0