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, 2020	Mar. 31, 2019
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ 13,900	$ (4,306)
Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(107)	19
Property
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	14,008	1,877
Property \| Actuarial methods
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	54,100
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		1,877
Property \| Large catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(32,091)	(12,292)
Property \| 2019 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(19,681)
Property \| 2018 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(14,200)
Property \| 2017 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(6,237)	(10,918)
Property \| Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	8,027	(1,374)
Property \| Small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	46,099	14,169
Property \| Other small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	46,099	14,169
Property \| Operating Segments
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	14,008	1,877
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(1)	(6,202)
Casualty and Specialty \| Actuarial methods
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(1)	(6,202)
Casualty and Specialty \| Operating Segments
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (1)	$ (6,202)