Fair value measurements (Level 3 rollforward) (Details) - Level 3 - Recurring - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Reconciliation of beginning and ending balances for all investments measured at fair value on recurring basis
Level 3 investments - beginning of period	$ 292,116	$ 102,033	$ 245,837	$ 50,248
Purchases	21,268	43,791	86,643	101,966
Sales		(2,929)	(2,389)	(4,918)
Settlements	(17,784)	(4,203)	(34,158)	(8,198)
Net realized losses				(11)
Change in net unrealized (losses) gains	(2,551)	(210)	(2,884)	(605)
Transfers into Level 3 during the period	0	0	0	0
Transfers out of Level 3 during the period	0	0	0	0
Level 3 investments - end of period	293,049	138,482	293,049	138,482
Bank loan portfolio
Reconciliation of beginning and ending balances for all investments measured at fair value on recurring basis
Level 3 investments - beginning of period	255,011	86,533	232,337	32,748
Purchases	8,885	43,791	50,988	101,966
Sales		(929)	(2,389)	(929)
Settlements	(17,784)	(4,203)	(34,033)	(8,198)
Net realized losses				0
Change in net unrealized (losses) gains	(2,964)	(210)	(3,755)	(605)
Transfers into Level 3 during the period	0	0	0	0
Transfers out of Level 3 during the period	0	0	0	0
Level 3 investments - end of period	243,148	124,982	243,148	124,982
Catastrophe bonds
Reconciliation of beginning and ending balances for all investments measured at fair value on recurring basis
Level 3 investments - beginning of period	37,105	15,500	13,500	17,500
Purchases	0	0	23,272	0
Sales		(2,000)	0	(3,989)
Settlements	0	0	(125)	0
Net realized losses				(11)
Change in net unrealized (losses) gains	413	0	871	0
Transfers into Level 3 during the period	0	0	0	0
Transfers out of Level 3 during the period	0	0	0	0
Level 3 investments - end of period	37,518	$ 13,500	37,518	$ 13,500
Asset-backed securities
Reconciliation of beginning and ending balances for all investments measured at fair value on recurring basis
Level 3 investments - beginning of period	0		0
Purchases	12,383		12,383
Sales			0
Settlements	0		0
Change in net unrealized (losses) gains	0		0
Transfers into Level 3 during the period	0		0
Transfers out of Level 3 during the period	0		0
Level 3 investments - end of period	$ 12,383		$ 12,383

Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of gain (loss) recognized in the income statement for financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)
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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of sale of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of settlement of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

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-Publisher FASB
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Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
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-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of transfers of financial instrument classified as an asset out of level 3 of the fair value hierarchy.

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-Publisher FASB
-Name Accounting Standards Codification
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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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