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SCHEDULE II CONDENSED FINANCIAL INFORMATION OF REGISTRANT PARENT COMPANY ONLY (BALANCE SHEETS) (Parenthetical) (Details) - USD ($)
Dec. 31, 2020
Sep. 19, 2020
Dec. 31, 2019
Condensed Financial Statements Captions [Line Items]      
Fixed maturities, amortized cost $ 6,945,600,000   $ 6,452,200,000
Preferred stock, par value $ 0.01   $ 0.01
Preferred stock, shares authorized 20,000,000.0   20,000,000.0
Preferred stock, issued 0   0
Common stock, par value $ 0.01   $ 0.01
Common stock, shares authorized 300,000,000.0   300,000,000.0
Common stock, shares issued 60,500,000   60,500,000
Treasury stock, shares 24,100,000   22,100,000
Senior debentures maturing September 1, 2030 [Member]      
Condensed Financial Statements Captions [Line Items]      
Aggregate principal amount $ 300,000,000.0    
Debt instrument interest rate 2.50%    
Subordinated debentures maturing March 30, 2053 [Member]      
Condensed Financial Statements Captions [Line Items]      
Aggregate principal amount   $ 175,000,000.0  
Debt instrument interest rate   6.35%  
The Hanover Insurance Group [Member]      
Condensed Financial Statements Captions [Line Items]      
Fixed maturities, amortized cost $ 369,500,000   $ 321,200,000
Preferred stock, par value $ 0.01   $ 0.01
Preferred stock, shares authorized 20,000,000.0   20,000,000.0
Preferred stock, issued 0   0
Common stock, par value $ 0.01   $ 0.01
Common stock, shares authorized 300,000,000.0   300,000,000.0
Common stock, shares issued 60,500,000   60,500,000
Treasury stock, shares 24,100,000   22,100,000
The Hanover Insurance Group [Member] | Senior debentures maturing September 1, 2030 [Member]      
Condensed Financial Statements Captions [Line Items]      
Aggregate principal amount $ 300,000,000.0    
Debt instrument interest rate 2.50%    
The Hanover Insurance Group [Member] | Subordinated debentures maturing March 30, 2053 [Member]      
Condensed Financial Statements Captions [Line Items]      
Aggregate principal amount $ 175,000,000.0    
Debt instrument interest rate 6.35%