Reserve for Claims and Claim Expenses (Schedule of Liability for Unpaid Claims and Claims Adjustment Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Net reserves as of January 1			$ 2,568,730	$ 2,632,519
Net incurred related to:
Current year	$ 1,209,261	$ 158,360	1,561,027	483,574
Prior years	12,435	(45,785)	(3,663)	(76,644)
Total net incurred	1,221,696	112,575	1,557,364	406,930
Net paid related to:
Current year			158,685	39,971
Prior years			396,411	391,047
Total net paid			555,096	431,018
Foreign exchange			33,011	11,899
Net reserves as of September 30	3,604,009	2,620,330	3,604,009	2,620,330
Reinsurance recoverable as of September 30	1,588,304	240,769	1,588,304	240,769
Gross reserves as of September 30	$ 5,192,313	$ 2,861,099	$ 5,192,313	$ 2,861,099