Segment reporting - Operating Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Gross premiums written	$ 87,587	$ 96,020
Gross premiums ceded	0	(9,975)
Net premiums written	87,587	86,045
Change in net unearned premium reserves	(14,325)	(52,504)
Net premiums earned	73,262	33,541
Loss and loss adjustment expenses incurred, net	46,259	18,638
Acquisition costs, net	25,431	13,073
General and administrative expenses	10,025	7,008
Total expenses	81,715	38,719
Net investment income	50,035	81,361
Other expenses	(787)	(670)
Income including non-controlling interests	40,795	75,513
Income attributable to non-controlling interests	(1,016)	(1,083)
Net income	39,779	74,430
Gross premiums as percentage of total gross premiums	100.00%	100.00%
Prospective Reinsurance Contract [Member]
Segment Reporting Information [Line Items]
Gross premiums written	85,485	96,020
Gross premiums as percentage of total gross premiums	97.60%	100.00%
Retroactive Reinsurance Contract [Member]
Segment Reporting Information [Line Items]
Gross premiums written	2,102	0
Gross premiums as percentage of total gross premiums	2.40%	0.00%
Corporate
Segment Reporting Information [Line Items]
Gross premiums written	0	0
Gross premiums ceded	0	0
Net premiums written	0	0
Change in net unearned premium reserves	0	0
Net premiums earned	0	0
Loss and loss adjustment expenses incurred, net	0	0
Acquisition costs, net	0	0
General and administrative expenses	3,382	1,407
Total expenses	3,382	1,407
Net investment income	42,693	75,068
Other expenses	0	0
Income including non-controlling interests	39,311	73,661
Income attributable to non-controlling interests	(825)	(504)
Net income	38,486	73,157
Property and Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	82,142	92,871
Gross premiums as percentage of total gross premiums	93.80%	96.70%
Property and Casualty Reinsurance | Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written	82,142	92,871
Gross premiums ceded	0	(9,975)
Net premiums written	82,142	82,896
Change in net unearned premium reserves	(9,841)	(50,253)
Net premiums earned	72,301	32,643
Loss and loss adjustment expenses incurred, net	46,259	18,638
Acquisition costs, net	25,399	12,991
General and administrative expenses	5,809	4,816
Total expenses	77,467	36,445
Underwriting loss	(5,166)	(3,802)
Net investment income	7,313	5,526
Other expenses	(787)	(670)
Income including non-controlling interests	1,360	1,054
Income attributable to non-controlling interests	0	0
Net income	1,360	1,054
Loss ratio	64.00%	57.10%
Acquisition cost ratio	35.10%	39.80%
Composite ratio	99.10%	96.90%
General and administrative expense ratio	8.00%	14.70%
Combined ratio	107.10%	111.60%
Catastrophe Risk Management
Segment Reporting Information [Line Items]
Gross premiums written	5,445	3,149
Gross premiums as percentage of total gross premiums	6.20%	3.30%
Catastrophe Risk Management | Operating Segments
Segment Reporting Information [Line Items]
Gross premiums written	5,445	3,149
Gross premiums ceded	0	0
Net premiums written	5,445	3,149
Change in net unearned premium reserves	(4,484)	(2,251)
Net premiums earned	961	898
Loss and loss adjustment expenses incurred, net	0	0
Acquisition costs, net	32	82
General and administrative expenses	834	785
Total expenses	866	867
Net investment income	29	767
Other expenses	0	0
Income including non-controlling interests	124	798
Income attributable to non-controlling interests	(191)	(579)
Net income	(67)	219
Property | Property and Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	6,881	350
Gross premiums as percentage of total gross premiums	7.90%	0.40%
Casualty | Property and Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	50,423	52,208
Gross premiums as percentage of total gross premiums	57.50%	54.30%
Specialty | Property and Casualty Reinsurance
Segment Reporting Information [Line Items]
Gross premiums written	$ 24,838	$ 40,313
Gross premiums as percentage of total gross premiums	28.40%	42.00%