Related Party Transactions Related Party Transactions (Narrative) (Details) - USD ($) $ in Millions	2 Months Ended	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Nephila Funds and Nephila Reinsurers [Member]
Related Party Transaction [Line Items]
Revenues attributable to management contracts with Nephila Funds and the Nephila Renisurers	$ 24.4
Syndicate 2357 [Member]
Related Party Transaction [Line Items]
Gross written premiums on Markel's program with Nephila		$ 322.1
Reinsurance recoverables from Syndicate 2357	179.8	179.8
Markel CATCo Re. [Member]
Related Party Transaction [Line Items]
Gross written premiums on behalf of Markel CATCo Re.		10.9	$ 9.7	$ 6.7
Maximum loss exposure on certain contracts	$ 250.0	$ 250.0