Loss and LAE Reserve Summary and Reconciliation to Consolidated Balance Sheet (Detail) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves		$ 11,871,250	[1]	$ 11,087,199	$ 10,799,200	$ 11,597,200
Reinsurance Recoverables on Unpaid Losses	[2]	(1,650,100)	[1]	(1,236,200)	(1,169,300)	(1,289,400)
Net Loss and LAE Reserves		10,221,200	[1]	$ 9,851,000	$ 9,629,900	$ 10,307,800
Eliminations
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	(67,800)
Reinsurance Recoverables on Unpaid Losses	[1]	67,800
Reinsurance Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	9,128,000
Reinsurance Recoverables on Unpaid Losses	[1]	(744,700)
Net Loss and LAE Reserves	[1]	8,383,300
Reinsurance Segment | Property
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	1,758,000
Reinsurance Recoverables on Unpaid Losses	[1]	(493,700)
Net Loss and LAE Reserves	[1]	1,264,300
Reinsurance Segment | Casualty & Other | Longer Tailed Lines of Business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	4,923,700
Reinsurance Recoverables on Unpaid Losses	[1]	(49,500)
Net Loss and LAE Reserves	[1]	4,874,200
Reinsurance Segment | Casualty & Other | Shorter Tailed Lines Of Business
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	2,446,300
Reinsurance Recoverables on Unpaid Losses	[1]	(201,500)
Net Loss and LAE Reserves	[1]	2,244,800
Insurance Segment
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	2,811,100
Reinsurance Recoverables on Unpaid Losses	[1]	(973,200)
Net Loss and LAE Reserves	[1]	1,837,900
Insurance Segment | RSUI
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	2,514,800
Reinsurance Recoverables on Unpaid Losses	[1]	(947,400)
Net Loss and LAE Reserves	[1]	1,567,400
Insurance Segment | RSUI | Property Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	581,500
Reinsurance Recoverables on Unpaid Losses	[1]	(225,900)
Net Loss and LAE Reserves	[1]	355,600
Insurance Segment | RSUI | Casualty Insurance
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	1,933,300
Reinsurance Recoverables on Unpaid Losses	[1]	(721,500)
Net Loss and LAE Reserves	[1]	1,211,800
Insurance Segment | CapSpecialty Incorporated
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Gross Loss and LAE Reserves	[1]	296,300
Reinsurance Recoverables on Unpaid Losses	[1]	(25,800)
Net Loss and LAE Reserves	[1],[3]	$ 270,500

[1] Includes unallocated LAE, which aggregate to 1.3 percent of gross loss and LAE reserves as of December 31, 2017. Net loss and LAE Reserves by component are shown in the preceding tables, and consolidated gross loss and LAE reserves is presented in the Consolidated Balance Sheets.

[2] Reinsurance recoverables in this table include only ceded loss and LAE reserves.

[3] The vast majority of the CapSpecialty's loss and LAE reserves relate to its casualty lines of business.