Investment information - Maturity profile of investments other than available for sale (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments, fair value option - cost or amortized cost	$ 2,048,103	$ 1,954,895
Investments, fair value option	1,732,085	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	1,966,319	1,871,330
Investments, fair value option	$ 1,643,312	$ 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	$ 368,381	$ 368,452
Investments, fair value option	$ 361,088	$ 370,479
Investments, fair value option percentage of fair value	22.00%	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	$ 809,886	$ 779,643
Investments, fair value option	$ 660,746	$ 742,960
Investments, fair value option percentage of fair value	40.20%	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	$ 582,501	$ 514,961
Investments, fair value option	$ 472,336	$ 495,416
Investments, fair value option percentage of fair value	28.70%	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		$ 514
Investments, fair value option		$ 533
Investments, fair value option percentage of fair value		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	$ 198,262	$ 200,361
Investments, fair value option	$ 140,613	$ 190,737
Investments, fair value option percentage of fair value	8.60%	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	$ 7,289	$ 7,399
Investments, fair value option	$ 8,529	$ 7,706
Investments, fair value option percentage of fair value	0.50%	0.40%