Investment information - Aging of available for sale securities in an unrealized loss position (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	$ 329,137	$ 167,848
Less than 12 months - gross unrealized losses	(43,425)	(1,753)
12 months or more - fair value	13,069	24,840
12 months or more - gross unrealized losses	(3,889)	(237)
Total - fair value	342,206	192,688
Total - gross unrealized losses	(47,314)	(1,990)
U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value		36,540
Less than 12 months - gross unrealized losses		(137)
12 months or more - fair value		0
12 months or more - gross unrealized losses		0
Total - fair value		36,540
Total - gross unrealized losses		(137)
Non-U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	120,115	51,779
Less than 12 months - gross unrealized losses	(6,564)	(1,027)
12 months or more - fair value	0	5,410
12 months or more - gross unrealized losses	0	(6)
Total - fair value	120,115	57,189
Total - gross unrealized losses	(6,564)	(1,033)
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	86,724	9,854
Less than 12 months - gross unrealized losses	(4,969)	(41)
12 months or more - fair value	0	0
12 months or more - gross unrealized losses	0	0
Total - fair value	86,724	9,854
Total - gross unrealized losses	(4,969)	(41)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	100,514	55,194
Less than 12 months - gross unrealized losses	(27,336)	(504)
12 months or more - fair value	13,069	19,430
12 months or more - gross unrealized losses	(3,889)	(231)
Total - fair value	113,583	74,624
Total - gross unrealized losses	(31,225)	(735)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Less than 12 months - fair value	21,784	14,481
Less than 12 months - gross unrealized losses	(4,556)	(44)
12 months or more - fair value	0	0
12 months or more - gross unrealized losses	0	0
Total - fair value	21,784	14,481
Total - gross unrealized losses	$ (4,556)	$ (44)