Short Duration Contracts - Reconciliation of claims development to liability (Details) - USD ($) $ in Thousands		Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		$ 13,445,271
Unpaid losses and loss adjustment expenses recoverable		6,278,825	$ 5,599,231	$ 4,314,855	$ 4,082,650
Lines other than short duration		102,283
Discounting		(60,536)
Unallocated claims adjustment expenses		266,100
Total reconciling items		307,847
Reserve for losses and loss adjustment expenses		20,031,943	$ 17,757,156	$ 16,513,929	$ 13,891,842
Other short duration lines not included
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		301,222
Unpaid losses and loss adjustment expenses recoverable	[1]	560,642
Other short duration lines not included | Loss portfolio transfer
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable	[1]	280,200
Total for short duration lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		6,278,825
Insurance | Property, energy, marine and aviation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		750,461
Unpaid losses and loss adjustment expenses recoverable		423,740
Insurance | Third party occurrence business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		3,184,995
Unpaid losses and loss adjustment expenses recoverable		1,761,871
Insurance | Third party claims-made business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,991,302
Unpaid losses and loss adjustment expenses recoverable		995,921
Insurance | Multi-line and other specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,215,721
Unpaid losses and loss adjustment expenses recoverable		173,051
Reinsurance | Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		2,323,825
Unpaid losses and loss adjustment expenses recoverable		684,663
Reinsurance | Property catastrophe
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		535,158
Unpaid losses and loss adjustment expenses recoverable		595,740
Reinsurance | Property excluding property catastrophe
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,010,604
Unpaid losses and loss adjustment expenses recoverable		206,867
Reinsurance | Marine and aviation
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		289,944
Unpaid losses and loss adjustment expenses recoverable		186,416
Reinsurance | Other specialty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,444,921
Unpaid losses and loss adjustment expenses recoverable		656,130
Mortgage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		397,118
Unpaid losses and loss adjustment expenses recoverable		37,531
Consolidating adjustments and eliminations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		$ (3,747)

[1] Includes unpaid loss and loss adjustment expenses recoverable of $280.2 million related to the loss portfolio transfer reinsurance agreements.