Short Duration Contracts - Reconciliation of claims development to liability (Details) - USD ($) $ in Thousands		Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		$ 11,886,330
Unpaid losses and loss adjustment expenses recoverable		5,599,231	$ 4,314,855	$ 4,082,650	$ 2,814,291
Lines other than short duration		76,240
Discounting		(55,575)
Unallocated claims adjustment expenses		225,033
Total reconciling items		245,698
Reserve for losses and loss adjustment expenses		17,757,156	$ 16,513,929	$ 13,891,842	$ 11,853,297
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]	390,100
Total for short duration lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		5,625,128
Operating segments | 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		441,459
Unpaid losses and loss adjustment expenses recoverable	[1]	887,457
Operating segments | 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		586,885
Unpaid losses and loss adjustment expenses recoverable		354,432
Operating segments | 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		2,885,557
Unpaid losses and loss adjustment expenses recoverable		1,537,549
Operating segments | 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		1,613,425
Unpaid losses and loss adjustment expenses recoverable		921,086
Operating segments | 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		1,125,841
Unpaid losses and loss adjustment expenses recoverable		199,485
Operating segments | Reinsurance | Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		2,106,507
Unpaid losses and loss adjustment expenses recoverable		592,879
Operating segments | 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		486,248
Unpaid losses and loss adjustment expenses recoverable		484,598
Operating segments | 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		724,034
Unpaid losses and loss adjustment expenses recoverable		118,939
Operating segments | 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		245,881
Unpaid losses and loss adjustment expenses recoverable		111,298
Operating segments | 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		981,805
Unpaid losses and loss adjustment expenses recoverable		373,292
Operating segments | Mortgage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		688,688
Unpaid losses and loss adjustment expenses recoverable		48,259
Consolidating adjustments and eliminations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		$ (4,146)

[1] Includes unpaid loss and loss adjustment expenses recoverable of $390.1 million related to the loss portfolio transfer reinsurance agreements.