Fair value - Level 3 rollforward (Details) - Recurring - Significant Unobservable Inputs (Level 3) - USD ($) $ in Thousands		3 Months Ended			6 Months Ended
		Jun. 30, 2020	Jun. 30, 2019		Jun. 30, 2020		Jun. 30, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance		$ 147,000	$ 171,738		$ 145,572		$ 164,767
Transfers in (out) of Level 3	[1]						(22,560)
Net Purchases (Sales)		(22,600)	170	[2]	(19,365)	[3]	30,777	[3]
Net Unrealized Gains (Losses)		13,095	2,492	[4]	11,288	[4]	1,815	[4]
Net Unrealized Foreign Exchange Gains (Losses)		0	311		0		(88)
Ending balance		137,495	174,711		137,495		174,711
Term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance		35,945	47,368		36,048		47,479
Transfers in (out) of Level 3	[1]						0
Net Purchases (Sales)		(7,184)	170	[2]	(7,258)	[3]	333	[3]
Net Unrealized Gains (Losses)		(108)	1,047	[4]	(137)	[4]	773	[4]
Net Unrealized Foreign Exchange Gains (Losses)		0	0		0		0
Ending balance		28,653	48,585		28,653		48,585
Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance		965	23,719		933		24,277
Transfers in (out) of Level 3	[1]						0
Net Purchases (Sales)		33	0	[2]	65	[3]	(90)	[3]
Net Unrealized Gains (Losses)		0	(110)	[4]	0	[4]	(179)	[4]
Net Unrealized Foreign Exchange Gains (Losses)		0	311		0		(88)
Ending balance		998	23,920		998		23,920
Fixed maturities | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance							22,560
Transfers in (out) of Level 3	[1]						(22,560)
Net Purchases (Sales)	[3]						0
Net Unrealized Gains (Losses)	[4]						0
Net Unrealized Foreign Exchange Gains (Losses)							0
Ending balance			0				0
Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Beginning balance		110,090	100,651		108,591		70,451
Transfers in (out) of Level 3	[1]						0
Net Purchases (Sales)		(15,449)	0	[2]	(12,172)	[3]	30,534	[3]
Net Unrealized Gains (Losses)		13,203	1,555	[4]	11,425	[4]	1,221	[4]
Net Unrealized Foreign Exchange Gains (Losses)		0	0		0		0
Ending balance		$ 107,844	$ 102,206		$ 107,844		$ 102,206

[1] During the six months ended June 30, 2019, the Company obtained pricing for an asset-backed security, for which pricing was not available as of December 31, 2018. As such, the security was transferred from Level 3 to Level 2 at its fair value as of December 31, 2018.

[2] For the three months ended June 30, 2020, the net purchases (sales) consisted of sales of equities of $25.1 million and sales, calls and redemptions of $7.2 million of term loans, offset by purchases of $9.6 million of equities and $33.0 thousand of corporate bonds. For the three months ended June 30, 2019, the net purchases (sales) consisted of purchases of $0.2 million of term loans, offset in part by calls and redemptions of $0.1 million.

[3] For the six months ended June 30, 2020, the net purchases (sales) consisted of sales of $25.1 million of equities and $7.3 million of sales, calls and redemptions of term loans, offset by purchases of $12.9 million of equities and $65.0 thousand of corporate bonds. For the six months ended June 30, 2019, the net purchases (sales) consisted of purchases of $37.8 million of equities and $0.5 million of term loans, offset in part by the sale of $7.3 million of equities, $0.1 million of redemptions of term loans and $0.1 million of corporate bonds.

[4] Realized and unrealized gains or losses on Level 3 investments are included in “realized and unrealized gain (loss) on investments” in the Company’s consolidated statements of income (loss).