Reserve for Claims and Claim Expenses (Reconciliation of the Disclosure of Incurred and Paid Claims Development to the Reserve for Claims and Claim Expenses) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	$ 2,568,730	$ 2,632,519	$ 1,345,816	$ 1,462,705
Reinsurance recoverable	279,564
Total gross reserve for claims and claim expenses	2,848,294	2,767,045	$ 1,412,510
Other
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	2,114
Reinsurance recoverable	23,280
Total gross reserve for claims and claim expenses	25,394	27,678
Property
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	584,036
Reinsurance recoverable	43,738
Total gross reserve for claims and claim expenses	627,774	706,201
Casualty and Specialty
Liability for Claims and Claims Adjustment Expense [Line Items]
Net reserve for claims and claim expenses	1,982,580
Reinsurance recoverable	212,546
Total gross reserve for claims and claim expenses	$ 2,195,126	$ 2,033,166