SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (STATEMENTS OF CASH FLOWS) (Details) - USD ($) $ in Millions	3 Months Ended											12 Months Ended
	Dec. 31, 2018		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Cash Flows From Operating Activities
Net income	$ 123.6		$ 100.4	$ 99.3	$ 67.7		$ 51.5		$ 11.1	$ 78.4	$ 45.2	$ 391.0		$ 186.2		$ 155.1
Adjustments to reconcile net income to net cash provided by operating activities:
Gain from sale of Chaucer business												(131.9)
Net loss from repayment of debt												1.9				88.3
Deferred income tax expense (benefit)												(23.4)		39.8		(13.2)
Change in expenses and taxes payable												4.6		(0.5)		12.7
Net cash provided by operating activities												551.3		704.6		743.4
Cash Flows From Investing Activities
Purchase of fixed maturities												(1,425.6)		(1,645.8)		(2,020.9)
Net cash received from sale of Chaucer business												635.7
Net cash provided by (used in) investing activities												271.3		(506.3)		(495.4)
Cash Flows From Financing Activities
Proceeds from debt borrowings, net																370.7
Repayment of debt												(11.6)				(475.4)
Repurchases of common stock												(57.7)		(37.2)		(105.6)
Dividends paid to shareholders												(94.3)		(86.8)		(80.4)
Proceeds from exercise of employee stock options												14.8		23.1		16.2
Other financing activities												(3.2)		(4.1)		(19.9)
Net cash used in financing activities												(171.0)		(111.7)		(300.8)
Net change in cash and cash equivalents												647.6		93.8		(56.2)
Cash and cash equivalents, beginning of year					297.9							297.9
Cash and cash equivalents, end of year	1,020.7						297.9					1,020.7		297.9
The Hanover Insurance Group [Member]
Cash Flows From Operating Activities
Net income												391.0		186.2		155.1
Adjustments to reconcile net income to net cash provided by operating activities:
Gain from sale of Chaucer business												(131.9)
Net realized investment losses												0.6		0.2
Net loss from repayment of debt												1.9				88.3
Equity in net income of subsidiaries												(288.4)		(196.1)		(207.0)
Dividends received from subsidiaries												86.4		55.1		228.1
Deferred income tax expense (benefit)												(5.3)		18.9		(39.0)
Change in expenses and taxes payable												59.7		(27.4)		6.6
Change in net receivable from subsidiaries												8.9		9.0		50.5
Other, net												(0.4)		(0.1)		7.3
Net cash provided by operating activities												122.5		45.8		289.9
Cash Flows From Investing Activities
Proceeds from disposals and maturities of fixed maturities												82.8		94.1		82.4
Purchase of fixed maturities												(4.0)
Net cash received from sale of Chaucer business												762.0
Net cash used for business acquisitions														(12.3)		(2.2)
Net cash provided by (used in) investing activities												840.8		81.8		80.2
Cash Flows From Financing Activities
Proceeds from debt borrowings, net																370.7
Proceeds from long-term intercompany borrowings																125.0
Net cash related to short-term intercompany borrowings												(19.8)				(102.7)
Repayment of debt												(11.6)				(571.9)
Repurchases of common stock												(57.7)		(37.2)		(105.6)
Dividends paid to shareholders												(94.3)		(86.8)		(80.4)
Proceeds from exercise of employee stock options												14.8		23.1		16.2
Other financing activities														(4.1)		(19.9)
Net cash used in financing activities												(168.6)		(105.0)		(368.6)
Net change in cash and cash equivalents												794.7		22.6		1.5
Cash and cash equivalents, beginning of year					$ 32.7	[1]					$ 10.1	32.7	[1]	10.1		8.6
Cash and cash equivalents, end of year	$ 827.4	[1]					$ 32.7	[1]				$ 827.4	[1]	$ 32.7	[1]	$ 10.1

[1] Included in 2018 were net cash proceeds of $762.0 million as a result of the sale of the Chaucer business to China Re on December 28, 2018.