Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
ReinsurancePremiumsForInsuranceCompaniesByProductSegmentLineItems
Gross Amount									$ 2,255,388	$ 1,903,576	$ 1,655,023
Ceded to Other Companies									1,193,359	1,512,380	3,548,264
Assumed earned premiums									4,427,006	4,447,862	3,842,836
Net Amount	$ 1,414,558	$ 1,428,400	$ 1,375,623	$ 1,270,454	$ 1,312,441	$ 1,230,771	$ 1,179,836	$ 1,116,010	5,489,035	4,839,058	1,949,595
Total Property And Liability Insurance Premiums Earned [Member]
ReinsurancePremiumsForInsuranceCompaniesByProductSegmentLineItems
Gross Amount									2,255,387	1,903,576	1,655,023
Ceded to Other Companies									1,193,359	1,512,380	3,548,264
Assumed earned premiums									4,427,006	4,447,862	3,842,836
Net Amount									$ 5,489,034	$ 4,839,058	$ 1,949,595
Assumed to Net									80.70%	91.90%	197.10%