Investment and other gains - net - Gross Realized Investment and Other Gains and Losses (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Investments, Debt and Equity Securities [Abstract]
Investment and other gains	$ 50,237	$ 1,597	$ 51,190	$ 2,763
Investment and other losses	(3,038)	(151)	(3,402)	(220)
Net unrealized investment (losses) gains recognized on equity securities still held at September 30	(294)	2,177	2,942	1,802
Investment and other gains (losses) – net	$ 46,905	$ 3,623	$ 50,730	$ 4,345

Amount of unrealized gain (loss) on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=116654751&loc=SL75117546-209714

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