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Reserve for Loss and Loss Expense (Reserve Loss Development) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years $ 742,722 $ 670,176 $ 641,174
Favorable or adverse loss development (39,192) (65,768) (69,009)
Favorable or unfavorable loss development [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years 39,200 65,800 69,000
Favorable or adverse loss development (39,200) (65,800) (69,000)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member]      
Favorable or adverse loss development 48,600 69,000 67,000
Favorable or unfavorable loss development [Member] | Property Insurance Product Line [Member]      
Favorable or adverse loss development 9,400 3,200 2,000
Favorable or unfavorable loss development [Member] | General Liability [Member]      
Favorable or adverse loss development (48,300) (45,000) (51,000)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member]      
Favorable or adverse loss development 35,600 25,300 2,400
Casualty Development 36,000 25,000  
Favorable or unfavorable loss development [Member] | Workers Compensation [Member]      
Favorable or adverse loss development (52,300) (56,000) (37,000)
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member]      
Favorable or adverse loss development 1,900 1,800 2,200
Favorable or unfavorable loss development [Member] | Commercial Property [Member]      
Favorable or adverse loss development 8,700 300 (3,000)
Favorable or unfavorable loss development [Member] | Homeowners [Member]      
Favorable or adverse loss development 400 1,700 1,500
Favorable or unfavorable loss development [Member] | Personal Automobile [Member]      
Favorable or adverse loss development 6,700 1,000 400
Favorable or unfavorable loss development [Member] | Excess And Surplus Operations [Member]      
Favorable or adverse loss development 10,000 6,000 16,000
Casualty Development 10,000    
Favorable or unfavorable loss development [Member] | Other Insurance Product Line [Member]      
Favorable or adverse loss development $ (1,900) $ (900) $ (500)