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Related Party Transactions Related Party Transactions (Narrative) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Related Party Transaction [Line Items]
Services and other revenues	$ 341,402	$ 200,495	$ 587,396	$ 393,839
Gross premium volume	2,373,192	2,247,147	4,696,016	4,502,189
Ceded premiums written	916,797	941,509	1,593,143	1,719,458
Reinsurance recoverables	5,472,349		5,472,349		$ 5,432,712
Syndicate 2357 and certain other Nephila Reinsurers [Member]
Related Party Transaction [Line Items]
Gross premium volume	122,900	141,200	213,400	234,200
Ceded premiums written	122,900	141,200	213,400	234,200
Reinsurance recoverables	236,700		236,700		$ 238,800
Hagerty Group, LLC [Member]
Related Party Transaction [Line Items]
Gross premium volume	154,000	128,600	254,900	212,900
Ceded premiums written	72,800	62,000	120,600	102,200
Insurance-linked Securities Operations [Member]
Related Party Transaction [Line Items]
Services and other revenues	54,600	50,200	107,700	103,600
Insurance-linked Securities Operations [Member] | Unconsolidated entities managed by Nephila and MCIM [Member]
Related Party Transaction [Line Items]
Services and other revenues	$ 46,000	$ 47,300	$ 89,900	$ 100,100

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