Reserve for Claims and Claim Expenses (Cumulative Number of Claims Reported) (Details) - Excess of Loss Reinsurance Contracts	Dec. 31, 2016 claim
2007 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	908
2007 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,388
2008 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,350
2008 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,368
2009 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	742
2009 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,082
2010 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	781
2010 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,030
2011 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,178
2011 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,356
2012 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	667
2012 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,253
2013 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	622
2013 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,187
2014 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	523
2014 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	1,353
2015 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	555
2015 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	988
2016 | Property
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	548
2016 | Casualty and Specialty
Claims Development [Line Items]
Cumulative number of reported claims (in claims)	365