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		9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ 12,435	$ (45,785)	$ (3,663)	$ (76,644)
Ogden Rate
Liability for Catastrophe Claims [Line Items]
Discount rate			(0.75%)		2.50%
Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ (155)	$ (808)	$ (710)	(805)
Property
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(16,968)	(37,512)
Property | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(843)	0
Property | Large and Small Catastrophe Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(17,811)	(37,512)
Property | Large catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			2,764	(8,015)
Property | April and May U.S. Tornadoes (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(4,163)	(3,396)
Property | Tohoku Earthquake and Tsunami (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			0	(5,894)
Property | New Zealand Earthquake (2010)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			5,001	8,710
Property | New Zealand Earthquake (2011)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			5,807	0
Property | Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(3,881)	(7,435)
Property | Small catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(20,575)	(29,497)
Property | Fort McMurray Wildfire (2016)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(6,386)	0
Property | Tianjin Explosion (2015)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(4,896)	(6,163)
Property | Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(9,293)	(23,334)
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			14,015	(38,327)
Casualty and Specialty | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(2,462)	(6,118)
Casualty and Specialty | Actuarial methods - actual reported claims lower than expected claims
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			(17,004)	(32,209)
Casualty and Specialty | Ogden Rate change
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years			$ 33,481	$ 0