SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Details) - USD ($) $ / shares in Units, $ in Millions		Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
Assets
Fair Value		$ 6,161.5		$ 5,749.3
Equity securities - at fair value		464.4		576.2
Cash and cash equivalents		1,020.7		297.9
Other assets		371.6		333.7
Total assets		12,399.7		15,469.6
Liabilities
Debt		777.9		786.9
Total liabilities		9,445.0		12,471.9
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		1,871.8		1,857.0
Accumulated other comprehensive income (loss)		(116.5)		107.6
Retained earnings		2,182.3		1,975.0
Treasury stock at cost (18.2 and 18.0 million shares)		(983.5)		(942.5)
Total shareholders’ equity		2,954.7		2,997.7		$ 2,857.5
Total liabilities and shareholders’ equity		12,399.7		15,469.6
Fixed maturities, amortized cost		$ 6,245.9		$ 5,657.1
Preferred stock, par value		$ 0.01		$ 0.01
Preferred stock, shares authorized		20,000,000		20,000,000
Preferred stock, issued		0		0
Common stock, par value		$ 0.01		$ 0.01
Common stock, shares authorized		300,000,000		300,000,000
Common stock, shares issued		60,500,000		60,500,000
Treasury stock, shares		18,200,000		18,000,000
The Hanover Insurance Group [Member]
Assets
Fair Value		$ 374.3		$ 280.9
Equity securities - at fair value		1.1		1.1
Cash and cash equivalents		827.4	[1]	32.7	[1]	$ 10.1	$ 8.6
Investments in subsidiaries		2,800.1		3,455.4
Net receivable from subsidiaries		25.0		31.3
Other assets	[2]	34.6		2.6
Total assets		4,062.5		3,804.0
Liabilities
Expenses and state taxes payable	[3]	275.2		7.9
Current income tax payable		46.9		3.2
Interest payable		7.8		8.1
Debt		777.9		787.1
Total liabilities		1,107.8		806.3
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		1,871.8		1,857.0
Accumulated other comprehensive income (loss)		(116.5)		107.6
Retained earnings		2,182.3		1,975.0
Treasury stock at cost (18.2 and 18.0 million shares)		(983.5)		(942.5)
Total shareholders’ equity		2,954.7		2,997.7
Total liabilities and shareholders’ equity		4,062.5		3,804.0
Fixed maturities, amortized cost		$ 380.5		$ 281.9
Preferred stock, par value		$ 0.01		$ 0.01
Preferred stock, shares authorized		20,000,000		20,000,000
Preferred stock, issued		0		0
Common stock, par value		$ 0.01		$ 0.01
Common stock, shares authorized		300,000,000		300,000,000
Common stock, shares issued		60,500,000		60,500,000
Treasury stock, shares		18,200,000		18,000,000

[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.

[2] In 2018, other assets include a $31.7 million receivable for estimated contingent consideration related to the sale of the Chaucer business.

[3] Included in 2018 was a payable related to a special dividend of $193.4 million declared on December 30, 2018.