SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF CASH FLOWS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Cash flows from operating activities
Net income	$ 35.3	$ 116.0	$ 422.8
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Deferred income tax benefit	(24.6)	(46.5)	25.0
Change in expenses and taxes payable	48.0	3.3	179.0
Other, net	1.3	(62.3)	(163.8)
Net cash provided by operating activities	361.7	722.3	823.7
Cash flows from investing activities
Proceeds from disposals and maturities of fixed maturities	168.8	489.0	485.0
Proceeds from disposals of equity securities and other investments	141.1	488.4	286.3
Purchase of fixed maturities	(1,059.8)	(1,960.2)	(2,112.7)
Purchase of equity securities and other investments	(70.8)	(124.3)	(190.2)
Net cash used in investing activities	(228.5)	(507.6)	(460.2)
Cash flows from financing activities
Proceeds from exercise of employee stock options	6.5	13.3	20.5
Dividends paid to shareholders	(117.2)	(108.9)	(102.2)
Repurchases of common stock		(30.8)	(162.6)
Net cash used in financing activities	(122.1)	(140.6)	(253.2)
Net change in cash and cash equivalents	11.1	74.1	110.3
Cash and cash equivalents, beginning of year	305.0	230.9	120.6
Cash and cash equivalents, end of year	316.1	305.0	230.9
The Hanover Insurance Group [Member]
Cash flows from operating activities
Net income	35.3	116.0	422.8
Adjustments to reconcile net income to net cash provided by (used in) operating activities:
Net realized investment (gains) losses	1.1	1.2	(1.2)
Equity in net income of subsidiaries	(62.2)	(144.6)	(444.7)
Dividends received from subsidiaries. net of capital contributed	0.7	12.2	89.5
Deferred income tax benefit	(0.4)	(2.5)	(2.0)
Change in expenses and taxes payable	(5.6)	4.7	(14.1)
Change in net receivable from subsidiaries	24.1	18.3	16.9
Other, net	1.4	3.2	4.8
Net cash provided by operating activities	(5.6)	8.5	72.0
Cash flows from investing activities
Proceeds from disposals and maturities of fixed maturities	111.4	155.0	238.7
Purchase of fixed maturities		(15.3)	(60.6)
Net cash used in investing activities	111.4	139.7	178.1
Cash flows from financing activities
Proceeds from exercise of employee stock options	6.5	13.3	20.5
Dividends paid to shareholders	(117.2)	(108.9)	(102.2)
Repurchases of common stock		(30.8)	(162.6)
Net cash used in financing activities	(110.7)	(126.4)	(244.3)
Net change in cash and cash equivalents	(4.9)	21.8	5.8
Cash and cash equivalents, beginning of year	43.2	21.4	15.6
Cash and cash equivalents, end of year	$ 38.3	$ 43.2	$ 21.4