Taxation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Current income tax expense [abstract]
Current income tax expense									$ 176,786	$ 235,982	$ 129,592
Deferred income tax expense (benefit) [abstract]
Deferred income tax (benefit) expense									(103,749)	2,774	(84,579)
Unrecognized tax expense (benefit) [abstract]
Unrecognized tax expense (benefit)									6,627	750	3,403
Income Tax Expense, Continuing Operations, by Jurisdiction [Abstract]
Total income tax expense	$ (3,000)	$ 17,000	$ (14,000)	$ 80,000	$ 53,000	$ 46,000	$ 78,000	$ 62,000	79,664	239,506	48,416
Income Before Taxes [Abstract]
Income before taxes									$ 186,814	$ 1,307,619	$ 721,858
Reconciliation of effective tax rate (% of income before taxes)
Expected tax rate									0.00%	0.00%	0.00%
Foreign taxes at local expected tax rates									58.30%	15.80%	5.10%
Impact of foreign exchange gains (losses)									1.10%	2.20%	(1.10%)
Unrecognized tax expense									3.50%	0.10%	0.50%
Tax-exempt income and expenses not deductible									(8.00%)	(2.20%)	(0.90%)
Impact of enacted changes in tax laws									0.30%	0.00%	1.80%
Foreign branch tax									(26.80%)	1.40%	(1.40%)
Ceding commissions									(0.70%)	1.80%	(0.40%)
Valuation allowance									15.20%	(0.60%)	1.30%
Other									(0.30%)	(0.20%)	1.80%
Actual tax rate									42.60%	18.30%	6.70%
US [Member]
Current income tax expense [abstract]
Current income tax expense									$ 81,066	$ 51,615	$ 55,993
Deferred income tax expense (benefit) [abstract]
Deferred income tax (benefit) expense									(59,624)	20,410	(13,693)
Unrecognized tax expense (benefit) [abstract]
Unrecognized tax expense (benefit)									0	0	(335)
Income Tax Expense, Continuing Operations, by Jurisdiction [Abstract]
Total income tax expense									21,442	72,025	41,965
Non-US [Member]
Current income tax expense [abstract]
Current income tax expense									95,720	184,367	73,599
Deferred income tax expense (benefit) [abstract]
Deferred income tax (benefit) expense									(44,125)	(17,636)	(70,886)
Unrecognized tax expense (benefit) [abstract]
Unrecognized tax expense (benefit)									6,627	750	3,738
Income Tax Expense, Continuing Operations, by Jurisdiction [Abstract]
Total income tax expense									58,222	167,481	6,451
Domestic (Bermuda)
Income Before Taxes [Abstract]
Income before taxes									(63,603)	686,538	611,900
Foreign
Income Before Taxes [Abstract]
Income before taxes									$ 250,417	$ 621,081	$ 109,958