Investment information - Summary of available for sale securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020		Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 749,835		$ 739,456
Gross Unrealized Gains	15,031		8,242
Gross Unrealized Losses	(47,314)		(1,990)
Fair Value	717,552		745,708
U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	256,191		282,076
Gross Unrealized Gains	8,635		1,708
Gross Unrealized Losses	0	[1]	(137)
Fair Value	264,826		283,647
Non-U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	151,000		155,834
Gross Unrealized Gains	4,040		2,326
Gross Unrealized Losses	(6,564)	[1]	(41)
Fair Value	148,476		158,119
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	169,735		145,555
Gross Unrealized Gains	2,296		614
Gross Unrealized Losses	(4,969)	[1]	(735)
Fair Value	167,062		145,434
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	144,808		129,456
Gross Unrealized Gains	0		3,530
Gross Unrealized Losses	(31,225)	[1]	(1,033)
Fair Value	113,583		131,953
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	26,341		24,776
Gross Unrealized Gains	0		18
Gross Unrealized Losses	(4,556)	[1]	(44)
Fair Value	21,785		24,750
Municipal government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	1,760		1,759
Gross Unrealized Gains	60		46
Gross Unrealized Losses	0	[1]	0
Fair Value	$ 1,820		$ 1,805

[1] Effective January 1, 2020, the Company adopted ASU 2016-13, and as a result any credit impairment losses on the Company’s available for sale securities are recorded as an allowance, subject to reversal. See Note 2. "Basis of presentation and significant accounting policies-(b) Recent accounting pronouncements-Issued and effective as of March 31, 2020 - Credit Losses" above for more information about ASU 2016-13. Included within the gross unrealized losses for corporate bonds is a credit allowance of $0.6 million for securities with an unrealized loss of $5.7 million as of March 31, 2020.