Investment income and other gains and losses - Gross Realized Investment and Other Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Available for sale securities gross realized gain (loss)
Realized gains	$ 548	$ 3,301	$ 1,392	$ 8,377
Realized losses	(1,595)	(48)	(2,828)	(3,671)
Realized gains (losses)	$ (1,047)	$ 3,253	$ (1,436)	$ 4,706

The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.3(a))
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 325
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75031664&loc=d3e22529-158502

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration