Segment reporting - Operating Segments (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Sep. 30, 2015 USD ($)	Jun. 30, 2015 USD ($)	Mar. 31, 2015 USD ($)	Dec. 31, 2016 USD ($) segment	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Segment Reporting [Abstract]
Number of operating segments | segment									1
Segment Reporting Information [Line Items]
Gross premiums written	$ 80,779	$ 142,573	$ 196,866	$ 197,156	$ 99,155	$ 205,583	$ 184,342	$ 213,334	$ 617,374	$ 702,414	$ 613,300
Gross premiums ceded	27	(927)	(1,425)	0	(24)	(375)	(1,425)	(52)	(2,325)	(1,876)	(150)
Net premiums written	80,806	141,646	195,441	197,156	99,131	205,208	182,917	213,282	615,049	700,538	613,150
Change in net unearned premium reserves	111,277	(13,463)	(62,319)	(60,354)	35,235	3,597	(62,339)	(74,207)	(24,859)	(97,714)	(168,618)
Net premiums earned	192,083	128,183	133,122	136,802	134,366	208,805	120,578	139,075	590,190	602,824	444,532
Loss and loss adjustment expenses incurred, net	122,110	85,015	104,131	84,676	98,855	158,537	76,053	81,746	395,932	415,191	283,147
Acquisition costs, net	76,854	45,127	48,482	51,687	38,552	50,509	47,498	54,657	222,150	191,216	137,206
General and administrative expenses	5,482	12,354	10,243	11,288	10,236	9,822	14,267	11,708	39,367	46,033	40,008
Net investment income									657,449	652,440	460,361
Net investment income (loss)	35,767	(88,356)	(86,346)	40,110	(61,553)	193,156	(38,611)	(64,918)	(98,825)	28,074	(85,582)
Other expenses	(2,161)	(347)	(3,173)	(2,706)	(2,928)	(670)	(2,315)	(2,701)	(8,387)	(8,614)	(7,395)
Interest expense	(2,068)	(2,069)	(2,046)	(2,048)	(2,074)	(2,074)	(2,052)	(1,036)	(8,231)	(7,236)	0
Net foreign exchange gains	5,162	3,905	8,068	2,386	2,396	746	(139)	193	19,521	3,196	0
Income tax (expense) benefit	272	(2,484)	(5,310)	1,929	(2,863)	7,781	(708)	(1,305)	(5,593)	2,905	(5,648)
Income (loss) including non-controlling interests	(46,925)	73,048	54,151	(51,398)	42,807	(197,436)	16,157	51,033	28,876	(87,439)	56,710
(Income) loss attributable to non-controlling interests	232	(967)	(775)	269	(614)	1,721	(495)	(563)	(1,241)	49	(6,315)
Net income (loss)	$ (46,693)	$ 72,081	$ 53,376	$ (51,129)	$ 42,193	$ (195,715)	$ 15,662	$ 50,470	27,635	(87,390)	50,395
Property and Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written									617,374	702,458	601,305
Catastrophe Risk Management
Segment Reporting Information [Line Items]
Gross premiums written									0	(44)	11,995
Operating Segments | Property and Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written									617,374	702,458	601,305
Gross premiums ceded									(2,325)	(1,876)	(150)
Net premiums written									615,049	700,582	601,155
Change in net unearned premium reserves									(24,859)	(97,766)	(168,858)
Net premiums earned									590,190	602,816	432,297
Loss and loss adjustment expenses incurred, net									395,932	415,041	283,180
Acquisition costs, net									222,150	191,217	136,154
General and administrative expenses									22,160	24,815	22,515
Net investment income									640,242	631,073	441,849
Net underwriting loss									(50,052)	(28,257)	(9,552)
Net investment income (loss)									(16,931)	10,810	(11,305)
Other expenses									(8,387)	(8,614)	(7,395)
Interest expense									0	0
Net foreign exchange gains									0	0
Income tax (expense) benefit									0	0	0
Income (loss) including non-controlling interests									(41,508)	(47,681)	(5,642)
(Income) loss attributable to non-controlling interests									0	0	0
Net income (loss)									$ (41,508)	$ (47,681)	$ (5,642)
Loss ratio									67.10%	68.90%	65.50%
Acquisition cost ratio									37.60%	31.70%	31.50%
Composite ratio									104.70%	100.60%	97.00%
General and administrative expense ratio									3.80%	4.10%	5.20%
Combined ratio									108.50%	104.70%	102.20%
Operating Segments | Catastrophe Risk Management
Segment Reporting Information [Line Items]
Gross premiums written									$ 0	$ (44)	$ 11,995
Gross premiums ceded									0	0	0
Net premiums written									0	(44)	11,995
Change in net unearned premium reserves									0	52	240
Net premiums earned									0	8	12,235
Loss and loss adjustment expenses incurred, net									0	150	(33)
Acquisition costs, net									0	(1)	1,052
General and administrative expenses									0	447	3,113
Net investment income									0	596	4,132
Net investment income (loss)									0	(69)	(1,227)
Other expenses									0	0	0
Interest expense									0	0
Net foreign exchange gains									0	0
Income tax (expense) benefit									0	0	0
Income (loss) including non-controlling interests									0	(519)	9,330
(Income) loss attributable to non-controlling interests									0	102	(4,725)
Net income (loss)									0	(417)	4,605
Corporate
Segment Reporting Information [Line Items]
Gross premiums written									0	0	0
Gross premiums ceded									0	0	0
Net premiums written									0	0	0
Change in net unearned premium reserves									0	0	0
Net premiums earned									0	0	0
Loss and loss adjustment expenses incurred, net									0	0	0
Acquisition costs, net									0	0	0
General and administrative expenses									17,207	20,771	14,380
Net investment income									17,207	20,771	14,380
Net investment income (loss)									(81,894)	17,333	(73,050)
Other expenses									0	0	0
Interest expense									(8,231)	(7,236)
Net foreign exchange gains									19,521	3,196
Income tax (expense) benefit									(5,593)	2,905	(5,648)
Income (loss) including non-controlling interests									70,384	(39,239)	53,022
(Income) loss attributable to non-controlling interests									(1,241)	(53)	(1,590)
Net income (loss)									$ 69,143	$ (39,292)	$ 51,432