Reserve for Claims and Claim Expenses (Reconciliation of the Disclosure of Incurred and Paid Claims Development to the Reserve for Claims and Claim Expenses) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	$ 3,493,778	$ 2,568,730	$ 2,632,519	$ 1,345,816
Reinsurance recoverable	1,586,630
Total gross reserve for claims and claim expenses	5,080,408	2,848,294	$ 2,767,045
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	673
Reinsurance recoverable	17,853
Total gross reserve for claims and claim expenses	18,526	25,394
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	1,343,290
Reinsurance recoverable	1,143,100
Total gross reserve for claims and claim expenses	2,486,390	627,774
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	2,149,815
Reinsurance recoverable	425,677
Total gross reserve for claims and claim expenses	$ 2,575,492	$ 2,195,126