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Income taxes (Components of tax expense) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosures [Line Items]
Income before taxes, loss from operating affiliate and (income) attributable to AlphaCat investors	$ 23,103	$ 136,929	$ 123,976	$ 206,846	$ 96,964	$ 104,298	$ 89,162	$ 190,170	$ 490,854	$ 480,594	$ 559,338
Components of income tax expense:
U.S.									2,042	739	21,257
Non-U.S.									711	6,028	26,179
Total current income tax expense									2,753	6,767	47,436
U.S.									(3,487)	1,360	(24,998)
Non-U.S.									(18,995)	(1,751)	(22,283)
Total deferred income tax benefit									(22,482)	(391)	(47,281)
U.S.									(1,445)	2,099	(3,741)
Non-U.S.									(18,284)	4,277	3,896
Total income tax (benefit) expense	$ (21,147)	$ 1,830	$ 1,706	$ (2,118)	$ (756)	$ 2,018	$ 2,549	$ 2,565	(19,729)	6,376	155
Bermuda
Income Tax Disclosures [Line Items]
Income before taxes, loss from operating affiliate and (income) attributable to AlphaCat investors									447,257	470,454	518,476
United Kingdom
Income Tax Disclosures [Line Items]
Income before taxes, loss from operating affiliate and (income) attributable to AlphaCat investors									4,308	13,621	10,824
United States
Income Tax Disclosures [Line Items]
Income before taxes, loss from operating affiliate and (income) attributable to AlphaCat investors									4,038	(4,176)	(11,752)
Switzerland
Income Tax Disclosures [Line Items]
Income before taxes, loss from operating affiliate and (income) attributable to AlphaCat investors									(1,084)	2,276	9,941
Canada
Income Tax Disclosures [Line Items]
Income before taxes, loss from operating affiliate and (income) attributable to AlphaCat investors									2,599	493	2,773
Other tax jurisdictions
Income Tax Disclosures [Line Items]
Income before taxes, loss from operating affiliate and (income) attributable to AlphaCat investors									$ 33,736	$ (2,074)	$ 29,076