Unpaid Losses And Loss Adjustment Expenses (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Deferred gain related to the completion of the Part VII transfer		$ (3,900)
Adverse (favorable) development related to the commutation of a property and casualty deposit contract			$ (11,700)
Net reserves for losses and loss adjustment expenses of acquired insurance companies	$ 0	57,493	0
Net losses and loss adjustment expenses	3,371,699	3,367,223	2,555,902
Adverse (favorable) development on prior years' loss reserves	$ (551,040)	$ (497,627)	(493,495)
IBNR reserves as a percentage of total unpaid losses and loss adjustment expenses	64.00%	64.00%
Asbestos-related reserves, gross loss incurred	$ 247,711	$ 274,527	323,904
Asbestos-related reserves, net loss incurred	83,048	104,661	111,604	$ 132,869
Asbestos and environmental net reserves for reported claims	74,400
Asbestos and environmental reserves incurred but not reported, net	8,700
Liability for asbestos and environmental claims, net claims paid since inception	647,700
Insurance [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses	2,596,057	2,442,344	2,009,426
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	12,088,244
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	6,928,416
Insurance [Member] | 2012 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	1,348,062	1,361,513	1,395,103	1,427,255	$ 1,489,026	$ 1,609,802	$ 1,369,219
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,150,793	1,117,221	1,052,859	937,643	780,067	567,450	233,371
Cumulative number of reported claims	127,000
Insurance [Member] | 2012 Accident Year [Member] | Alterra [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance						256,400
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance						36,800
Insurance [Member] | 2013 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 1,387,804	1,428,733	1,462,652	1,525,750	1,694,879	1,735,667
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 1,099,985	1,037,635	949,370	779,023	571,548	271,439
Cumulative number of reported claims	87
Insurance [Member] | 2013 Accident Year [Member] | Alterra [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance						313,300
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance						29,500
Insurance [Member] | 2017 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 2,160,771	2,326,966
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 990,931	438,289
Cumulative number of reported claims	116
Insurance [Member] | 2018 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	$ 2,452,747
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	$ 496,812
Cumulative number of reported claims	147
Reinsurance [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses	$ 775,642	924,879	546,476
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	4,264,423
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	1,880,409
Reinsurance [Member] | 2012 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	446,722	454,839	457,038	485,374	507,023	550,343	72,903
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	288,772	263,811	231,541	183,943	128,769	64,460	$ 4,049
Reinsurance [Member] | 2012 Accident Year [Member] | Alterra [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance						474,200
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance						52,600
Reinsurance [Member] | 2013 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	506,609	543,963	534,148	547,828	578,994	586,074
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	331,630	301,714	268,761	$ 209,893	$ 155,515	71,503
Reinsurance [Member] | 2013 Accident Year [Member] | Alterra [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance						536,200
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance						$ 68,900
Reinsurance [Member] | 2017 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	939,604	906,216
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	359,659	158,049
Reinsurance [Member] | 2018 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Ultimate incurred losses and allocated loss adjustment expenses, net of reinsurance	760,161
Cumulative paid losses and allocated loss adjustment expenses, net of reinsurance	87,614
Asbestos [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Asbestos-related reserves, gross loss incurred	188,300
Asbestos-related reserves, net loss incurred	63,400
Gen Liab Workers Comp Prof Liab and Marine And Energy Insurance Segment Surety And Marine And Energy Reinsurance Segment [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Adverse (favorable) development on prior years' loss reserves	$ (424,100)
Gen Liab Prof Liab Workers Comp Marine And Energy Insurance Segment Surety And Marine And Energy Reinsurance Segment [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Adverse (favorable) development on prior years' loss reserves		(422,900)
Ogden Rate Change [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Adverse (favorable) development on prior years' loss reserves		85,000
General liability property And Marine And Energy Insurance segment And Property lines Reinsurance segment [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Adverse (favorable) development on prior years' loss reserves			(418,000)
Development On Specified Medical And Medical Malpractice [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Adverse (favorable) development on prior years' loss reserves			$ 71,200
Personal Lines [Member] | Insurance [Member] | 2012 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Cumulative number of reported claims	66
Personal Lines [Member] | Insurance [Member] | 2013 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Cumulative number of reported claims	17
Personal Lines [Member] | Insurance [Member] | 2017 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Cumulative number of reported claims	24
Personal Lines [Member] | Insurance [Member] | 2018 Accident Year [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Cumulative number of reported claims	46
2018 Catastrophes [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses, net of assumed reinstatement premiums	$ 287,300
Net losses and loss adjustment expenses	292,800
Net assumed reinstatement premiums	(5,400)
Reinsurance recoverables	$ 244,100
2017 Catastrophes [Member]
Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Net losses and loss adjustment expenses, net of assumed reinstatement premiums		565,300
Net losses and loss adjustment expenses		585,400
Net assumed reinstatement premiums		(20,100)
Reinsurance recoverables		$ 490,300