Reserve for Claims and Claim Expenses (Cumulative Number of Claims Reported) (Details) - Excess of Loss Reinsurance Contracts	Dec. 31, 2019 claim
Accident Year 2010 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	835
Accident Year 2010 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,215
Accident Year 2011 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,388
Accident Year 2011 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,964
Accident Year 2012 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	922
Accident Year 2012 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	2,263
Accident Year 2013 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	799
Accident Year 2013 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	2,712
Accident Year 2014 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	744
Accident Year 2014 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	3,421
Accident Year 2015 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	758
Accident Year 2015 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	3,783
Accident Year 2016 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,089
Accident Year 2016 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	4,135
Accident Year 2017 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	2,215
Accident Year 2017 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	3,294
Accident Year 2018 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	2,050
Accident Year 2018 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	2,148
Accident Year 2019 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	861
Accident Year 2019 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	816