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Reserve for Losses and Loss Expenses (Reserve Loss Development) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years $ 641,174 $ 592,062 $ 572,434
Favorable or adverse loss development (69,009) (59,269) (25,525)
Property Development     11,000
Favorable or unfavorable loss development [Member]      
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years 69,000 59,300 25,500
Favorable or adverse loss development (69,000) (59,300) (25,500)
Property Development 2,000 11,100  
Casualty Development 67,000 48,200 14,500
Favorable or unfavorable loss development [Member] | General Liability [Member]      
Favorable or adverse loss development (51,000) (43,900) (20,000)
Favorable or unfavorable loss development [Member] | Commercial Automobile [Member]      
Favorable or adverse loss development 2,400 (4,100) (4,500)
Favorable or unfavorable loss development [Member] | Workers' Compensation [Member]      
Favorable or adverse loss development (37,000) 0 23,500
Favorable or unfavorable loss development [Member] | Business Owners' Policies [Member]      
Favorable or adverse loss development 2,200 1,900 (9,500)
Favorable or unfavorable loss development [Member] | Commercial Property [Member]      
Favorable or adverse loss development (3,000) (2,100) (7,500)
Favorable or unfavorable loss development [Member] | Homeowners [Member]      
Favorable or adverse loss development 1,500 (4,000) (2,500)
Favorable or unfavorable loss development [Member] | Personal Automobile [Member]      
Favorable or adverse loss development 400 (10,800) (3,000)
Favorable or unfavorable loss development [Member] | Excess And Surplus Operations [Member]      
Favorable or adverse loss development $ 15,500 $ 3,700 $ (2,000)