Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	$ 537,890	$ 513,128	$ 488,525
Reserve for Losses and Loss Expenses		10,669,150	10,369,701
Unearned Premiums	3,283,300	3,137,133	3,026,732
Net Premiums Earned	6,293,348	6,040,609	5,744,418
Net Investment Income	564,163	512,645	600,885
Loss and Loss Expenses	3,845,800	3,656,270	3,490,567
Amortization of Deferred Policy Acquisition Cost	1,155,954	1,102,492	1,053,397
Other Operating Costs and Expenses	1,239,665	1,187,258	1,104,059
Net Premiums Written	6,423,913	6,189,515	5,996,947
Reinsurance Recoverable
Supplementary Insurance Information, by Segment [Line Items]
Reserve for Losses and Loss Expenses	11,197,195
Operating Segments | Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	446,875	430,664	408,161
Reserve for Losses and Loss Expenses	9,747,934	9,248,306	8,856,715
Unearned Premiums	2,993,063	2,854,461	2,727,257
Net Premiums Earned	5,652,903	5,431,500	5,074,308
Net Investment Income	455,139	410,457	484,039
Loss and Loss Expenses	3,449,857	3,300,283	3,075,007
Amortization of Deferred Policy Acquisition Cost	964,838	929,982	884,018
Other Operating Costs and Expenses	968,611	931,586	880,087
Net Premiums Written	5,775,913	5,591,397	5,345,663
Operating Segments | Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	91,015	82,464	80,364
Reserve for Losses and Loss Expenses	1,449,261	1,420,844	1,512,986
Unearned Premiums	290,237	282,672	299,475
Net Premiums Earned	640,445	609,109	670,110
Net Investment Income	78,967	74,226	88,821
Loss and Loss Expenses	395,943	355,987	415,560
Amortization of Deferred Policy Acquisition Cost	191,116	172,510	169,379
Other Operating Costs and Expenses	57,552	59,986	58,315
Net Premiums Written	648,000	598,118	651,284
Corporate and adjustments
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	0	0	0
Reserve for Losses and Loss Expenses	0	0	0
Unearned Premiums	0	0	0
Net Premiums Earned	0	0	0
Net Investment Income	30,057	27,962	28,025
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Costs and Expenses	213,502	195,686	165,657
Net Premiums Written	$ 0	$ 0	$ 0