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Reserve for Losses and Loss Expenses (Reserve Loss Development) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years $ 670,176 $ 641,174 $ 592,062
Favorable or adverse loss development (65,768) (69,009) (59,269)
Favorable or unfavorable loss development [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years 65,800 69,000 59,300
Favorable or adverse loss development (65,800) (69,000) (59,300)
Favorable or unfavorable loss development [Member] | Casualty Insurance Product Line [Member]      
Favorable or adverse loss development 69,000 67,000 48,200
Favorable or unfavorable loss development [Member] | Property Insurance Product Line [Member]      
Favorable or adverse loss development 3,200 2,000 11,100
Favorable or unfavorable loss development [Member] | General Liability [Member]      
Favorable or adverse loss development (45,000) (51,000) (43,900)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member]      
Favorable or adverse loss development 25,300 2,400 (4,100)
Favorable or unfavorable loss development [Member] | Workers Compensation [Member]      
Favorable or adverse loss development (56,000) (37,000) 0
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member]      
Favorable or adverse loss development 1,800 2,200 1,900
Favorable or unfavorable loss development [Member] | Commercial Property [Member]      
Favorable or adverse loss development 300 (3,000) (2,100)
Favorable or unfavorable loss development [Member] | Homeowners [Member]      
Favorable or adverse loss development 1,700 1,500 (4,000)
Favorable or unfavorable loss development [Member] | Personal Automobile [Member]      
Favorable or adverse loss development 1,000 400 (10,800)
Favorable or unfavorable loss development [Member] | Excess And Surplus Operations [Member]      
Favorable or adverse loss development 7,100 15,500 3,700
Casualty Development 6,000    
Favorable or unfavorable loss development [Member] | Other Insurance Product Line [Member]      
Favorable or adverse loss development $ (2,000) $ 0 $ 0