Reserve for Claims and Claim Expenses (Prior Year Development of the Reserve for Net Claims and Claim Expenses) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2022	Mar. 31, 2021
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (17,833)	$ (9,032)
Property
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(16,758)	(5,162)
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	(16,758)	(5,162)
Property \| Large catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(14,290)	(14,192)
Property \| 2021 Weather-Related Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	19,190
Property \| 2020 Weather-Related Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,272)
Property \| 2019 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(4,807)	(4,742)
Property \| 2018 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(3,800)	(3,263)
Property \| 2017 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(18,000)	(5,508)
Property \| Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(1,601)	(679)
Property \| Small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(2,468)	9,030
Property \| Other small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(2,468)	9,030
Property \| Operating Segments
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(16,758)	(5,162)
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(1,075)	(3,870)
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	(1,075)	(3,870)
Casualty and Specialty \| Operating Segments
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (1,075)	$ (3,870)