Acquisitions (Consideration Transferred and Recognized Amounts of Identifiable Assets and Liabilities) (Details) - USD ($) $ in Thousands		6 Months Ended
	Apr. 02, 2015	Sep. 30, 2015	Dec. 31, 2014
Recognized amounts of identifiable assets acquired and liabilities assumed:
Goodwill		$ 354,781	$ 278,258
Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc [Member]
Consideration:
Cash for initial consideration	$ 193,889
Receivable for post-closing adjustments	(31,160)
Fair value of consideration transferred	162,729
Recognized amounts of identifiable assets acquired and liabilities assumed:
Fixed maturity investments	245,997
Cash and cash equivalents	46,853
Insurance balances receivable	113,922
Prepaid reinsurance	17,491
Reinsurance recoverable	57,350
Intangible assets and VOBA	127,704
Other assets	10,041
Reserve for losses and loss expenses	(310,198)
Unearned premiums	(150,530)
Reinsurance balances payable	(35,734)
Net deferred tax liabilities	(16,488)
Accounts payable and accrued liabilities	(19,918)
Total identifiable net assets acquired	86,490
Goodwill	76,239
Total net assets acquired	162,729
Business Combination, Acquisition Related Costs		$ 8,948
Value Of Business Acquired [Member] | Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangible assets and VOBA	37,104
Intangible Assets Excluding Voba [Member] | Hong Kong And Singapore Operations Of Royal Sun Alliance Insurance Plc [Member]
Recognized amounts of identifiable assets acquired and liabilities assumed:
Intangible assets and VOBA	$ 90,600