Short Duration Contracts - Reconciliation of claims development to liability (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		$ 9,536,526
Unpaid losses and loss adjustment expenses recoverable		4,082,650	$ 2,814,291	$ 2,464,910	$ 2,083,575
Lines other than short duration		55,989
Discounting		(22,012)
Unallocated claims adjustment expenses		188,055
Total reconciling items		222,032
Reserve for losses and loss adjustment expenses		13,891,842	$ 11,853,297	$ 11,383,792	$ 10,200,960
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	[1]	1,514,373
Unpaid losses and loss adjustment expenses recoverable	[2],[3]	1,472,777
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	[2],[3]	319,300
Total for short duration lines
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		4,133,284
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		340,714
Unpaid losses and loss adjustment expenses recoverable		213,004
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		2,264,839
Unpaid losses and loss adjustment expenses recoverable		1,105,669
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		1,225,121
Unpaid losses and loss adjustment expenses recoverable		728,511
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		887,227
Unpaid losses and loss adjustment expenses recoverable		189,613
Reinsurance | Casualty
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		1,707,696
Unpaid losses and loss adjustment expenses recoverable		532,387
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		113,053
Unpaid losses and loss adjustment expenses recoverable		282,910
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		458,581
Unpaid losses and loss adjustment expenses recoverable		48,554
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		131,895
Unpaid losses and loss adjustment expenses recoverable		28,893
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		637,789
Unpaid losses and loss adjustment expenses recoverable		206,022
Mortgage
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liabilities for losses and loss adjustment expenses, net of reinsurance		255,238
Unpaid losses and loss adjustment expenses recoverable		21,875
Consolidating adjustments and eliminations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Unpaid losses and loss adjustment expenses recoverable		(696,931)
Other | 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	[1]	$ 1,100,000

[1] Includes net outstanding liabilities of $1.1 billion for the ‘other’ segment

[2] $319.3 million related to the loss portfolio transfer reinsurance agreement.

[3] Includes unpaid loss and loss adjustment expenses recoverable