Fair value - Fair value hierarchy (Details) - Recurring - USD ($) $ in Thousands		Mar. 31, 2020		Dec. 31, 2019
Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		$ 571,218		$ 617,060
Total liabilities measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other underwriting derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investment derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		0	[1]	0	[2]
Quoted Prices in Active Markets for Identical Assets (Level 1) | Term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Corporate bonds | Payable for securities sold short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		265,310		285,500
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Non-U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | Municipal government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		296,158		318,012
Quoted Prices in Active Markets for Identical Assets (Level 1) | Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		9,750		13,548
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other underwriting derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value				0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Investment derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]			0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other investments measured at net asset value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[2]	0		0
Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		1,763,906		1,917,859
Total liabilities measured at fair value		54,116		66,514
Significant Other Observable Inputs (Level 2) | Other underwriting derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		314
Significant Other Observable Inputs (Level 2) | Investment derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		23,726	[1]	257	[2]
Significant Other Observable Inputs (Level 2) | Term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		871,054		1,025,886
Significant Other Observable Inputs (Level 2) | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		407,175		371,540
Significant Other Observable Inputs (Level 2) | Fixed maturities | Corporate bonds | Payable for securities sold short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		30,076		66,257
Significant Other Observable Inputs (Level 2) | Fixed maturities | U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		113		109
Significant Other Observable Inputs (Level 2) | Fixed maturities | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		254,196		336,171
Significant Other Observable Inputs (Level 2) | Fixed maturities | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		30,314		32,456
Significant Other Observable Inputs (Level 2) | Fixed maturities | Non-U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		149,858		133,409
Significant Other Observable Inputs (Level 2) | Fixed maturities | Municipal government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		2,073		2,184
Significant Other Observable Inputs (Level 2) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		47,703		11,291
Significant Other Observable Inputs (Level 2) | Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		1,420		2,998
Significant Other Observable Inputs (Level 2) | Other underwriting derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value				148
Significant Other Observable Inputs (Level 2) | Investment derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]			1,667
Significant Other Observable Inputs (Level 2) | Other investments measured at net asset value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[2]	0		0
Significant Unobservable Inputs (Level 3)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		147,000		145,572
Total liabilities measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Other underwriting derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		0
Significant Unobservable Inputs (Level 3) | Investment derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		0	[1]	0	[2]
Significant Unobservable Inputs (Level 3) | Term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		35,945		36,048
Significant Unobservable Inputs (Level 3) | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		965		933
Significant Unobservable Inputs (Level 3) | Fixed maturities | Corporate bonds | Payable for securities sold short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Fixed maturities | U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Fixed maturities | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Fixed maturities | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Fixed maturities | Non-U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Fixed maturities | Municipal government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		0		0
Significant Unobservable Inputs (Level 3) | Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		110,090		108,591
Significant Unobservable Inputs (Level 3) | Other underwriting derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value				0
Significant Unobservable Inputs (Level 3) | Investment derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]			0
Significant Unobservable Inputs (Level 3) | Other investments measured at net asset value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[2]	0		0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		2,512,806		2,710,952
Total liabilities measured at fair value		54,116		66,514
Estimated Fair Value | Other underwriting derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		314
Estimated Fair Value | Investment derivative liabilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		23,726	[1]	257	[2]
Estimated Fair Value | Term loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		906,999		1,061,934
Estimated Fair Value | Fixed maturities | Corporate bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		408,140		372,473
Estimated Fair Value | Fixed maturities | Corporate bonds | Payable for securities sold short
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total liabilities measured at fair value		30,076		66,257
Estimated Fair Value | Fixed maturities | U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		265,423		285,609
Estimated Fair Value | Fixed maturities | Asset-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		254,196		336,171
Estimated Fair Value | Fixed maturities | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		30,314		32,456
Estimated Fair Value | Fixed maturities | Non-U.S. government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		149,858		133,409
Estimated Fair Value | Fixed maturities | Municipal government and government agency bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		2,073		2,184
Estimated Fair Value | Short-term investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		343,861		329,303
Estimated Fair Value | Equities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		121,260		125,137
Estimated Fair Value | Other underwriting derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value				148
Estimated Fair Value | Investment derivative assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]			1,667
Estimated Fair Value | Other investments measured at net asset value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[2]	$ 30,682		$ 30,461

[1] Investment derivative assets and liabilities represent the fair value of total return swaps, which are recorded in “other liabilities” in the consolidated balance sheets as of March 31, 2020

[2] In accordance with applicable accounting guidance, other investments that are measured at fair value using the net asset value practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented in the consolidated balance sheets.

[3] Investment derivative assets and liabilities represent the fair value of total return swaps, which are recorded in “other assets” and “other liabilities,” respectively, in the consolidated balance sheets as of December 31, 2019.