Reserve for Claims and Claim Expenses (Additional Information) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	$ 6,593,052,000	$ 3,704,050,000	$ 3,493,778,000	$ 2,568,730,000
Claims and claim expenses recovered	(1,124,757,000)	(1,458,518,000)	(1,558,960,000)
Reinsurance balances payable
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Aggregate deposit liabilities	9,000,000.0	10,300,000
Aggregate deposit assets
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Aggregate deposit assets	0	0
Deposit Contracts, Underwriting Risk Only
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Claims and claim expenses recovered	0	200,000	200,000
Deposit Contracts, Timing Risk Only
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Increase in other income related to assumed reinsurance contracts	1,300,000	$ 11,200,000	$ 3,700,000
Property
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	2,288,762,000
Casualty and Specialty
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	$ 4,304,061,000