Investment information - Maturity profile of investments other than available for sale (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 2,052,907	$ 1,954,895
Investments, fair value option	1,886,676	1,898,091
Fixed income and short term investments
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	1,973,371	1,871,330
Investments, fair value option	$ 1,793,636	$ 1,807,831
Investments, fair value option percentage of fair value	100.00%	100.00%
Fixed income and short term investments | Due in one year or less
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 423,881	$ 368,452
Investments, fair value option	$ 415,624	$ 370,479
Investments, fair value option percentage of fair value	23.20%	20.50%
Fixed income and short term investments | Due after one year through five years
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 797,528	$ 779,643
Investments, fair value option	$ 708,229	$ 742,960
Investments, fair value option percentage of fair value	39.50%	41.10%
Fixed income and short term investments | Due after five years through ten years
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 538,257	$ 514,961
Investments, fair value option	$ 495,214	$ 495,416
Investments, fair value option percentage of fair value	27.60%	27.40%
Fixed income and short term investments | Due after ten years
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 8,136	$ 514
Investments, fair value option	$ 7,480	$ 533
Investments, fair value option percentage of fair value	0.40%	0.00%
Fixed income and short term investments | No single maturity date | Asset-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 198,619	$ 200,361
Investments, fair value option	$ 157,925	$ 190,737
Investments, fair value option percentage of fair value	8.80%	10.60%
Fixed income and short term investments | No single maturity date | Mortgage-backed securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 6,950	$ 7,399
Investments, fair value option	$ 9,164	$ 7,706
Investments, fair value option percentage of fair value	0.50%	0.40%