Reserve for Claims and Claim Expenses (Prior Year Development of the Reserve for Net Claims and Claim Expenses) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (270,749)	$ (40,996)	$ (164,126)
Discount rate | Ogden Rate
Liability for Catastrophe Claims [Line Items]
Cash surrender value, measurement input		(0.0075)	0.025
Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(197)	$ (1,583)	$ (1,110)
Property
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(221,290)	(45,596)	(104,876)
Property | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,682)	842
Property | Large and small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(215,608)	(46,438)	(104,876)
Property | Large catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(182,029)	(7,640)	(33,926)
Property | Storm Sandy (2012)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years		(4,395)	(10,849)
Property | April and May U.S. Tornadoes (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years		(4,177)
Property | Thailand Floods (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(15,131)
Property | Tohoku Earthquake and Tsunami (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(7,314)
Property | New Zealand Earthquake (2010)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years		4,061	6,904
Property | New Zealand Earthquake (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years		5,807	1,987
Property | Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(9,517)	(8,936)	(9,523)
Property | Small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(33,579)	(38,798)	(70,950)
Property | 2017 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(172,512)
Property | Tianjin Explosion (2015)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years		(8,002)	(5,686)
Property | Fort McMurray Wildfire (2016)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years		(6,364)
Property | Other small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(33,579)	(24,432)	(65,264)
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(49,262)	6,183	(58,140)
Casualty and Specialty | Actuarial methods - actual reported claims less than expected claims
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(41,476)	(24,836)	(52,601)
Casualty and Specialty | Ogden Rate change
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	0	33,481	0
Casualty and Specialty | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (7,786)	$ (2,462)	$ (5,539)