Investment information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	$ 302,177	$ 167,848
Less than 12 months - gross unrealized losses	(20,659)	(1,753)
12 months or more - fair value	16,135	24,840
12 months or more - gross unrealized losses	(2,224)	(237)
Total - fair value	318,312	192,688
Total - gross unrealized losses	(22,883)	(1,990)
U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	28,550	36,540
Less than 12 months - gross unrealized losses	(4)	(137)
12 months or more - fair value	0	0
12 months or more - gross unrealized losses	0	0
Total - fair value	28,550	36,540
Total - gross unrealized losses	(4)	(137)
Non-U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	121,648	51,779
Less than 12 months - gross unrealized losses	(6,591)	(1,027)
12 months or more - fair value	0	5,410
12 months or more - gross unrealized losses	0	(6)
Total - fair value	121,648	57,189
Total - gross unrealized losses	(6,591)	(1,033)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	24,696	9,854
Less than 12 months - gross unrealized losses	(2,224)	(41)
12 months or more - fair value	0	0
12 months or more - gross unrealized losses	0	0
Total - fair value	24,696	9,854
Total - gross unrealized losses	(2,224)	(41)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	105,868	55,194
Less than 12 months - gross unrealized losses	(9,343)	(504)
12 months or more - fair value	16,135	19,430
12 months or more - gross unrealized losses	(2,224)	(231)
Total - fair value	122,003	74,624
Total - gross unrealized losses	(11,567)	(735)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	21,415	14,481
Less than 12 months - gross unrealized losses	(2,497)	(44)
12 months or more - fair value	0	0
12 months or more - gross unrealized losses	0	0
Total - fair value	21,415	14,481
Total - gross unrealized losses	$ (2,497)	$ (44)