SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Details) - USD ($) $ in Millions		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Assets
Fair Value		$ 7,454.4	$ 6,687.1
Equity securities - at fair value		598.5	575.7
Cash and cash equivalents		120.6	215.7
Other assets		445.7	402.4
Total assets		13,443.7	12,490.5
Liabilities
Debt		780.8	653.4
Total liabilities		10,241.5	9,574.3
Shareholders’ Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued		0.6	0.6
Additional paid-in capital		1,857.4	1,837.3
Accumulated other comprehensive income		372.5	152.6
Retained earnings		2,668.0	2,410.9
Treasury stock at cost (24.1 and 22.1 million shares)		(1,696.3)	(1,485.2)
Total shareholders’ equity		3,202.2	2,916.2	$ 2,954.7
Total liabilities and shareholders’ equity		13,443.7	12,490.5
The Hanover Insurance Group [Member]
Assets
Fair Value		381.3	328.5
Equity securities - at fair value		1.1	1.1
Cash and cash equivalents		15.6	12.2
Investments in subsidiaries		3,718.3	3,352.2
Net receivable from subsidiaries		25.2	25.1
Other assets		2.9	3.2
Total assets		4,144.4	3,722.3
Liabilities
Expenses and state taxes payable		23.2	16.4
Current income tax payable		2.7	3.5
Interest payable		10.5	7.8
Debt	[1]	905.8	778.4
Total liabilities		942.2	806.1
Shareholders’ Equity
Preferred stock, par value $0.01 per share; 20.0 million shares authorized; none issued
Common stock, par value $0.01 per share; 300.0 million shares authorized; 60.5 million shares issued		0.6	0.6
Additional paid-in capital		1,857.4	1,837.3
Accumulated other comprehensive income		372.5	152.6
Retained earnings		2,668.0	2,410.9
Treasury stock at cost (24.1 and 22.1 million shares)		(1,696.3)	(1,485.2)
Total shareholders’ equity		3,202.2	2,916.2
Total liabilities and shareholders’ equity		$ 4,144.4	$ 3,722.3

[1] In 2020, the Company issued $300.0 million aggregate principal amount of 2.50% unsecured senior debentures. With the proceeds, THG retired its outstanding $175.0 million principal amount of 6.35% subordinated unsecured debentures. (See Note 6 – “Debt and Credit Arrangements” in Notes to Consolidated Financial Statements).