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, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	$ 6,593,052	$ 3,704,050	$ 3,493,778	$ 2,568,730
Reinsurance recoverable	2,791,297
Total gross reserve for claims and claim expenses	9,384,349	6,076,271	$ 5,080,408
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	229
Reinsurance recoverable	211
Total gross reserve for claims and claim expenses	440	$ 4,624
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	2,288,762
Reinsurance recoverable	1,785,088
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	4,304,061
Reinsurance recoverable	$ 1,005,998