Reserve for Claims and Claim Expenses (Cumulative Number of Claims Reported) (Details) - Excess of Loss Reinsurance Contracts	Dec. 31, 2018 claim
Accident Year 2009 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	743
Accident Year 2009 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,132
Accident Year 2010 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	787
Accident Year 2010 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,081
Accident Year 2011 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,186
Accident Year 2011 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,411
Accident Year 2012 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	679
Accident Year 2012 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,448
Accident Year 2013 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	629
Accident Year 2013 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,557
Accident Year 2014 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	553
Accident Year 2014 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	2,040
Accident Year 2015 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	606
Accident Year 2015 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,992
Accident Year 2016 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	845
Accident Year 2016 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,650
Accident Year 2017 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,812
Accident Year 2017 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,028
Accident Year 2018 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,014
Accident Year 2018 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	277