SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
The Hanover Insurance Group [Member]
Supplementary Insurance Information By Segment [Line Items]
Investment assets transferred to the parent company to settle dividend balances	$ 185.6	$ 70.1	$ 179.8

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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