Reserve for Claims and Claim Expenses (Prior Year Development of the Reserve for Net Claims and Claim Expenses) (Details) - USD ($)	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (9,032,000)	$ 13,900,000
Property
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,162,000)	13,901,000
Property \| Actuarial methods - actual reported claims less than expected claims
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years		54,000,000.0
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	(5,162,000)
Property \| Large catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(14,192,000)	(32,091,000)
Property \| 2019 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(4,742,000)	(19,681,000)
Property \| 2018 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(3,263,000)	(14,200,000)
Property \| 2017 Large Loss Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,508,000)	(6,237,000)
Property \| Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(679,000)	8,027,000
Property \| Small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	9,030,000	45,992,000
Property \| Other small catastrophe events and attritional loss movements
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	9,030,000	45,992,000
Property \| Operating Segments
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,162,000)	13,901,000
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(3,870,000)	(1,000.0)
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	(3,870,000)	(1,000)
Casualty and Specialty \| Operating Segments
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (3,870,000)	$ (1,000)