Reserve for Claims and Claim Expenses (Cumulative Number of Claims Reported) (Details) - Excess of Loss Reinsurance Contracts	Dec. 31, 2017 claim
Accident Year 2008 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,355
Accident Year 2008 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,395
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)	1,112
Accident Year 2010 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	784
Accident Year 2010 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,052
Accident Year 2011 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,184
Accident Year 2011 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,367
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)	1,354
Accident Year 2013 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	628
Accident Year 2013 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,409
Accident Year 2014 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	549
Accident Year 2014 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,771
Accident Year 2015 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	591
Accident Year 2015 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,569
Accident Year 2016 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	818
Accident Year 2016 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,114
Accident Year 2017 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,173
Accident Year 2017 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	313