Reserve for Losses and Loss Expenses Reserves for Losses and Loss Expenses (Rollforward of Reserves for Losses and Loss Expenses) (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Liability for Catastrophe Claims [Line Items]
Gross reserves for losses and loss expenses, at beginning of year	$ 3,691,719	$ 3,517,728
Less: reinsurance recoverable on unpaid losses and loss expenses, at beginning of year	611,200	551,019
Net reserves for losses and loss expenses, at beginning of year	3,080,519	2,966,709
Incurred losses and loss expenses for claims occurring in the current year	1,037,079	955,347
Total incurred losses and loss expenses	1,003,618	911,881
Paid losses and loss expenses for claims occurring in the current year	314,686	275,623
Paid losses and loss expenses for claims occurring in the prior years	581,186	547,067
Total paid losses and loss expenses	895,872	822,690
Net reserves for losses and loss expenses, at end of period	3,188,265	3,055,900
Add: reinsurance recoverable on unpaid losses and loss expenses, at end of period	647,535	630,686
Gross reserves for losses and loss expenses, at end of period	3,835,800	3,686,586
Change in Historical Claim Rate Experience [Member]
Liability for Catastrophe Claims [Line Items]
Prior Year Claims and Claims Adjustment Expense	(33,461)	(43,466)
Change in Historical Claim Rate Experience [Member] | General Liability [Member]
Liability for Catastrophe Claims [Line Items]
Prior Year Claims and Claims Adjustment Expense	(48,300)	(33,000)
Change in Historical Claim Rate Experience [Member] | Workers Compensation [Member]
Liability for Catastrophe Claims [Line Items]
Prior Year Claims and Claims Adjustment Expense	(29,300)	(36,000)
Change in Historical Claim Rate Experience [Member] | Commercial Automobile [Member]
Liability for Catastrophe Claims [Line Items]
Prior Year Claims and Claims Adjustment Expense	26,000	20,000
Change in Historical Claim Rate Experience [Member] | Excess And Surplus Operations [Member]
Liability for Catastrophe Claims [Line Items]
Prior Year Claims and Claims Adjustment Expense	10,000	$ 3,000
Change in Historical Claim Rate Experience [Member] | Personal Automobile [Member]
Liability for Catastrophe Claims [Line Items]
Prior Year Claims and Claims Adjustment Expense	$ 4,000