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		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ 29,006	$ (1,604)
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	$ (328)	(82)
Property
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(928)	(5,928)
Property | Large and Small Catastrophe Events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(928)	(5,928)
Property | Large catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	2,586	3,778
Property | Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	2,586	3,778
Property | Small catastrophe events
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(3,514)	(9,706)
Property | Fort McMurray Wildfire (2016)
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	(5,850)	0
Property | Other
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	2,336	(9,706)
Casualty and Specialty
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	30,262	4,406
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	(5,999)	4,406
Casualty and Specialty | Ogden Rate change
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	33,481	0
Casualty and Specialty | Actuarial assumption changes
Liability for Catastrophe Claims [Line Items]
Net claims and claim expenses incurred – prior accident years	$ 2,780	$ 0