Fair value - Fair value hierarchy (Details) - Recurring - USD ($) $ in Thousands		Jun. 30, 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		$ 598,133		$ 617,060
Total liabilities measured at fair value		6,123		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		217,347		285,500
Quoted Prices in Active Markets for Identical Assets (Level 1) | Fixed maturities | U.S. government and government agency 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		6,123
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		368,384		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		12,402		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		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	[2]			0
Quoted Prices in Active Markets for Identical Assets (Level 1) | Other investments measured at net asset value (2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	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		1,869,581		1,917,859
Total liabilities measured at fair value		30,101		66,514
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		6,935	[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		846,907		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		547,103		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		23,166		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		112		109
Significant Other Observable Inputs (Level 2) | Fixed maturities | U.S. government and government agency 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
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		288,252		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		31,182		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		151,124		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		2,117		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		1,682		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		1,096		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		6		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	[2]			1,667
Significant Other Observable Inputs (Level 2) | Other investments measured at net asset value (2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	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		137,495		145,572
Total liabilities measured at fair value		0		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		28,653		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		998		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 | U.S. government and government agency 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
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		107,844		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		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	[2]			0
Significant Unobservable Inputs (Level 3) | Other investments measured at net asset value (2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	0		0
Estimated Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value		2,639,351		2,710,952
Total liabilities measured at fair value		36,224		66,514
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		6,935	[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		875,560		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		548,101		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		23,166		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		217,459		285,609
Estimated Fair Value | Fixed maturities | U.S. government and government agency 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		6,123
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		288,252		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		31,182		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		151,124		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		2,117		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		370,066		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		121,342		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		6		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	[2]			1,667
Estimated Fair Value | Other investments measured at net asset value (2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets measured at fair value	[3]	$ 34,142		$ 30,461

[1] n 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.

[2] 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.

[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 June 30, 2020.