SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF CASH FLOWS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income	$ 358.7	$ 425.1	$ 391.0
Adjustments to reconcile net income to net cash provided by operating activities:
(Gain) loss from sale of Chaucer business		1.2	(131.9)
Loss on repayment of debt	6.2		28.2
Deferred income tax benefit	(23.0)	12.5	(23.4)
Change in expenses and taxes payable	67.5	11.0	4.6
Other, net	(24.7)	(14.7)	(31.7)
Net cash provided by operating activities	707.6	602.9	551.3
Cash flows from investing activities
Proceeds from disposals of equity securities and other investments	172.8	250.3	158.4
Purchase of fixed maturities	(1,829.1)	(1,588.3)	(1,425.6)
Purchase of equity securities and other investments	(255.1)	(332.2)	(178.3)
Net cash received from sale of Chaucer business		34.7	635.7
Net cash provided by (used in) investing activities	(608.8)	(311.9)	271.3
Cash flows from financing activities
Proceeds from exercise of employee stock options	6.3	14.4	14.8
Proceeds from debt borrowings, net	296.4
Dividends paid to shareholders	(99.5)	(386.2)	(94.3)
Repayment of debt	(175.8)	(151.1)	(11.6)
Repurchases of common stock	(212.8)	(563.6)	(57.7)
Net cash used in financing activities	(193.9)	(1,099.3)	(171.0)
Cash and cash equivalents, beginning of year	215.7	1,020.7	297.9
Cash and cash equivalents, end of year	120.6	215.7	1,020.7
The Hanover Insurance Group [Member]
Cash flows from operating activities
Net income	358.7	425.1	391.0
Adjustments to reconcile net income to net cash provided by operating activities:
(Gain) loss from sale of Chaucer business		2.1	(131.9)
Net realized investment (gains) losses	(0.4)	(5.3)	0.6
Loss on repayment of debt	6.2		1.9
Equity in net income of subsidiaries	(387.2)	(432.3)	(288.4)
Dividends received from subsidiaries	60.3	69.9	86.4
Deferred income tax benefit	(8.5)	(13.9)	(5.3)
Change in expenses and taxes payable	5.6	(107.8)	59.7
Change in net receivable from subsidiaries	15.5	11.6	8.9
Other, net	4.4	3.9	(0.4)
Net cash provided by operating activities	54.6	(46.7)	122.5
Cash flows from investing activities
Proceeds from disposals and maturities of fixed maturities	166.6	447.9	82.8
Proceeds from disposals of equity securities and other investments		121.9
Purchase of fixed maturities	(32.4)	(316.1)	(4.0)
Purchase of equity securities and other investments		(121.9)
Net cash received from sale of Chaucer business		35.1	762.0
Net cash provided by (used in) investing activities	134.2	166.9	840.8
Cash flows from financing activities
Proceeds from exercise of employee stock options	6.3	14.4	14.8
Proceeds from debt borrowings, net	296.4
Dividends paid to shareholders	(99.5)	(386.2)	(94.3)
Repayment of debt	(175.8)		(11.6)
Repurchases of common stock	(212.8)	(563.6)	(57.7)
Net cash related to short-term intercompany borrowings			(19.8)
Net cash used in financing activities	(185.4)	(935.4)	(168.6)
Net change in cash and cash equivalents	3.4	(815.2)	794.7
Cash and cash equivalents, beginning of year	12.2	827.4	32.7
Cash and cash equivalents, end of year	$ 15.6	$ 12.2	$ 827.4