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, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	$ 3,704,050	$ 3,493,778	$ 2,568,730	$ 2,632,519
Reinsurance recoverable	2,372,221
Total gross reserve for claims and claim expenses	6,076,271	5,080,408	$ 2,848,294
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	368
Reinsurance recoverable	4,256
Total gross reserve for claims and claim expenses	4,624	$ 18,526
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	1,373,125
Reinsurance recoverable	1,713,129
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	2,330,557
Reinsurance recoverable	$ 654,836