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Reserve for Claims and Claim Expenses (Prior Year Development of the Reserve for Net Claims and Claim Expenses) (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years $ (156,229) $ (45,104) $ (187,564) $ (16,098)
Ogden Rate | Maximum        
Liability for Catastrophe Claims [Line Items]        
Discount rate       2.50%
Ogden Rate | Minimum        
Liability for Catastrophe Claims [Line Items]        
Discount rate       (0.75%)
Other        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years $ (88) $ (227) (70) $ (555)
Property        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (170,707) (24,800)
Property | Actuarial methods        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (32,265)  
Property | Actuarial assumption changes        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (5,682) 843
Property | Attritional net claims and claim expenses        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (37,947)  
Property | Large and Small Catastrophe Events        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (132,760) (25,643)
Property | Large catastrophe events        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (156,213) 1,851
Property | 2017 Catastrophe Events        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (156,213)  
Property | Other        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years       (3,864)
Property | New Zealand Earthquake (2011)        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years       5,807
Property | New Zealand Earthquake (2010)        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years       4,061
Property | April and May U.S. Tornadoes (2011)        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years       (4,153)
Property | Small catastrophe events        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     23,453 (27,494)
Property | Fort McMurray Wildfire (2016)        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years       (5,848)
Property | Tianjin Explosion (2015)        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years       (4,896)
Property | U.S. PCS 13/14 Wind and Thunderstorm (2013)        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years       (3,906)
Property | Other        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     23,453 (12,844)
Casualty and Specialty        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (16,787) 9,257
Casualty and Specialty | Actuarial methods        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (9,001) (21,762)
Casualty and Specialty | Actuarial assumption changes        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     (7,786) (2,462)
Casualty and Specialty | Ogden Rate change        
Liability for Catastrophe Claims [Line Items]        
Net claims and claim expenses incurred – prior accident years     $ 0 $ 33,481