Investment information - Summary of available for sale securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020		Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	$ 698,897			$ 739,456
Gross Unrealized Gains	14,211			8,242
Gross Unrealized Losses	(22,883)			(1,990)
Fair Value	690,225			745,708
Allowance for expected credit losses on available for sale securities	400		$ 600
U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	215,127			282,076
Gross Unrealized Gains	1,741			1,708
Gross Unrealized Losses	(4)	[1]		(137)
Fair Value	216,864			283,647
Non-U.S. government and government agency bonds
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	151,335			129,456
Gross Unrealized Gains	4,998			3,530
Gross Unrealized Losses	(6,591)	[1]		(1,033)
Fair Value	149,742			131,953
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	164,417			155,834
Gross Unrealized Gains	7,215			2,326
Gross Unrealized Losses	(2,224)	[1]		(41)
Fair Value	169,408			158,119
Unrealized losses on investments with credit losses	2,600
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	141,760			145,555
Gross Unrealized Gains	134			614
Gross Unrealized Losses	(11,567)	[1]		(735)
Fair Value	130,327			145,434
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Cost or Amortized Cost	24,498			24,776
Gross Unrealized Gains	17			18
Gross Unrealized Losses	(2,497)	[1]		(44)
Fair Value	22,018			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	106			46
Gross Unrealized Losses	0	[1]		0
Fair Value	$ 1,866			$ 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 June 30, 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 $0.4 million for securities with an unrealized loss of $2.6 million as of June 30, 2020.