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, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (26,855)	$ (270,749)	$ (40,996)
Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(40)	(197)	(1,583)
Property
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(2,933)	(221,290)	(45,596)
Property | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	9,245	(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	(12,178)	(215,608)	(46,438)
Property | Large catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(17,557)	(182,029)	(7,640)
Property | 2017 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(101,572)	(172,512)
Property | New Zealand Earthquake (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(7,497)		5,807
Property | Tohoku Earthquake and Tsunami (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,198)
Property | Storm Sandy (2012)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(4,395)
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 | New Zealand Earthquake (2010)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	47,071		4,061
Property | 2018 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	81,555
Property | Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(31,916)	(9,517)	(8,936)
Property | Small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	5,379	(33,579)	(38,798)
Property | Tianjin Explosion (2015)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(8,002)
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	5,379	(33,579)	(24,432)
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(23,882)	(49,262)	6,183
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	(52,796)	(41,476)	(24,836)
Casualty and Specialty | Ogden Rate change
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	0	0	33,481
Casualty and Specialty | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ 28,914	$ (7,786)	$ (2,462)
High | Discount rate | Ogden Rate
Liability for Catastrophe Claims [Line Items]
Cash surrender value, measurement input			0.025
Low | Discount rate | Ogden Rate
Liability for Catastrophe Claims [Line Items]
Cash surrender value, measurement input			(0.0075)