SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF CASH FLOWS) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Cash flows from operating activities
Net income	$ 116.0	$ 418.7	$ 358.7
Adjustments to reconcile net income to net cash provided by operating activities:
Loss on repayment of debt			6.2
Deferred income tax benefit	(46.5)	23.9	(23.0)
Change in expenses and taxes payable	3.3	179.0	67.5
Other, net	(62.3)	(163.8)	(24.7)
Net cash provided by operating activities	722.3	823.7	707.6
Cash flows from investing activities
Proceeds from disposals and maturities of fixed maturities	489.0	485.0	264.1
Proceeds from disposals of equity securities and other investments	488.4	286.3	172.8
Purchase of fixed maturities	(1,960.2)	(2,112.7)	(1,829.1)
Purchase of equity securities and other investments	(124.3)	(190.2)	(255.1)
Net cash used in investing activities	(507.6)	(460.2)	(608.8)
Cash flows from financing activities
Proceeds from exercise of employee stock options	13.3	20.5	6.3
Proceeds from debt borrowings, net			296.4
Dividends paid to shareholders	(108.9)	(102.2)	(99.5)
Repayment of debt			(175.8)
Repurchases of common stock	(30.8)	(162.6)	(212.8)
Net cash used in financing activities	(140.6)	(253.2)	(193.9)
Net change in cash and cash equivalents	74.1	110.3	(95.1)
Cash and cash equivalents, beginning of year	230.9	120.6	215.7
Cash and cash equivalents, end of year	305.0	230.9	120.6
The Hanover Insurance Group [Member]
Cash flows from operating activities
Net income	116.0	418.7	358.7
Adjustments to reconcile net income to net cash provided by operating activities:
Net realized investment (gains) losses	1.2	(1.2)	(0.4)
Loss on repayment of debt			6.2
Equity in net income of subsidiaries	(144.6)	(440.6)	(387.2)
Dividends received from subsidiaries	12.2	89.5	60.3
Deferred income tax benefit	(2.5)	(2.0)	(8.5)
Change in expenses and taxes payable	4.7	(14.1)	5.6
Change in net receivable from subsidiaries	18.3	16.9	15.5
Other, net	3.2	4.8	4.4
Net cash provided by operating activities	8.5	72.0	54.6
Cash flows from investing activities
Proceeds from disposals and maturities of fixed maturities	155.0	238.7	166.6
Purchase of fixed maturities	(15.3)	(60.6)	(32.4)
Net cash used in investing activities	139.7	178.1	134.2
Cash flows from financing activities
Proceeds from exercise of employee stock options	13.3	20.5	6.3
Proceeds from debt borrowings, net			296.4
Dividends paid to shareholders	(108.9)	(102.2)	(99.5)
Repayment of debt			(175.8)
Repurchases of common stock	(30.8)	(162.6)	(212.8)
Net cash used in financing activities	(126.4)	(244.3)	(185.4)
Net change in cash and cash equivalents	21.8	5.8	3.4
Cash and cash equivalents, beginning of year	21.4	15.6	12.2
Cash and cash equivalents, end of year	$ 43.2	$ 21.4	$ 15.6