Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	$ 507,549	$ 537,890	$ 513,128
Reserve for Losses and Loss Expenses		11,197,195	10,669,150
Unearned Premiums	3,290,180	3,283,300	3,137,133
Net Premiums Earned	6,311,419	6,293,348	6,040,609
Net Investment Income	575,788	564,163	512,645
Loss and Loss Expenses	4,002,348	3,845,800	3,656,270
Amortization of Deferred Policy Acquisition Cost	1,111,489	1,155,954	1,102,492
Other Operating Costs and Expenses	1,325,443	1,239,665	1,187,258
Net Premiums Written	6,260,508	6,423,913	6,189,515
Reinsurance Recoverable
Supplementary Insurance Information, by Segment [Line Items]
Reserve for Losses and Loss Expenses	11,670,408
Operating Segments | Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	435,967	442,317	426,036
Reserve for Losses and Loss Expenses	9,820,258	9,445,210	8,857,342
Unearned Premiums	3,039,343	2,975,060	2,834,691
Net Premiums Earned	5,706,443	5,618,842	5,393,166
Net Investment Income	436,178	431,489	386,801
Loss and Loss Expenses	3,516,996	3,430,139	3,279,219
Amortization of Deferred Policy Acquisition Cost	929,793	964,064	918,901
Other Operating Costs and Expenses	1,026,545	954,858	927,095
Net Premiums Written	5,715,871	5,743,620	5,555,437
Operating Segments | Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Policy Acquisition Cost	71,582	95,573	87,092
Reserve for Losses and Loss Expenses	1,850,150	1,751,985	1,811,808
Unearned Premiums	250,837	308,240	302,442
Net Premiums Earned	604,976	674,506	647,443
Net Investment Income	91,146	102,617	97,882
Loss and Loss Expenses	485,352	415,661	377,051
Amortization of Deferred Policy Acquisition Cost	181,696	191,890	183,591
Other Operating Costs and Expenses	44,349	71,305	64,477
Net Premiums Written	544,637	680,293	634,078
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	48,464	30,057	27,962
Loss and Loss Expenses	0	0	0
Amortization of Deferred Policy Acquisition Cost	0	0	0
Other Operating Costs and Expenses	254,549	213,502	195,686
Net Premiums Written	$ 0	$ 0	$ 0