Segment information (Segment reporting by segment - Asset Management activities) (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Share compensation expenses		$ 10,031	$ 9,443	$ 11,146	$ 9,491	$ 10,442	$ 10,501	$ 10,727	$ 11,237	$ 40,111	$ 42,907	$ 38,341
Finance expenses		15,871	14,523	14,209	13,943	14,630	14,521	14,166	15,203	58,546	58,520	74,742
Tax expense (benefit)		(412)	(2,632)	(987)	(3,549)	(21,147)	1,830	1,706	(2,118)	(7,580)	(19,729)	6,376
Foreign exchange (losses) gains		(283)	(1,404)	(7,329)	1,569	(901)	(766)	6,286	6,245	(7,447)	10,864	(8,731)
Net investment income		48,960	44,458	44,241	40,214	38,153	43,514	39,257	29,461	177,873	150,385	127,824
Gross premiums written		$ 443,323	$ 523,856	$ 792,902	$ 1,190,857	$ 339,454	$ 372,418	$ 764,042	$ 1,172,791	2,950,938	2,648,705	2,557,506
Operating Segments | Asset Management Segment
Segment Reporting Information [Line Items]
Third party management fees										20,349	18,771	19,661
Related party management fees										2,150	3,329	5,309
Total fee revenues										22,499	22,100	24,970
General and administrative expenses										12,904	10,233	12,115
Share compensation expenses										389	249	580
Finance expenses										137	947	9,312
Tax expense (benefit)										8	90	0
Foreign exchange (losses) gains										(7)	(19)	16
Total expenses										13,445	11,538	21,991
Income before investment (loss) income from funds and sidecars										9,054	10,562	2,979
Net investment income										(25,149)	20,579	19,176
Asset Management segment (loss) income										(16,095)	31,141	22,155
Gross premiums written										312,819	270,402	176,126
Operating Segments | Asset Management Segment | AlphaCat ILS Funds - Lower Risk
Segment Reporting Information [Line Items]
Net investment income	[1],[2]									(3,102)	8,901	7,491
Gross premiums written	[1]									128,295	112,222	91,363
Operating Segments | Asset Management Segment | AlphaCat ILS Funds - Higher Risk
Segment Reporting Information [Line Items]
Net investment income	[1],[2]									(22,662)	7,471	8,428
Gross premiums written	[1]									157,976	140,022	34,228
Operating Segments | Asset Management Segment | BetaCat ILS Funds
Segment Reporting Information [Line Items]
Net investment income	[2]									536	3,623	1,702
Operating Segments | Asset Management Segment | PaCRe
Segment Reporting Information [Line Items]
Net investment income	[2]									0	(23)	(3,949)
Operating Segments | Asset Management Segment | AlphaCat Sidecars
Segment Reporting Information [Line Items]
Net investment income	[2]									79	607	5,504
Gross premiums written										(181)	(341)	45,755
Operating Segments | Asset Management Segment | AlphaCat Direct
Segment Reporting Information [Line Items]
Gross premiums written	[3]									$ 26,729	$ 18,499	$ 4,780

[1] Lower risk AlphaCat ILS funds have a maximum permitted portfolio expected loss of less than 7%, whereas higher risk AlphaCat ILS funds have a maximum permitted portfolio expected loss of 7% or greater. The maximum permitted portfolio expected loss represents the average annual loss over the set of simulation scenarios divided by the total limit.

[2] The investment income (loss) from funds and sidecars is based on equity accounting.

[3] AlphaCat Direct includes direct investments from a third party investor in AlphaCat Re.