Investment information - Credit quality of investments (Details) - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019
Investments by Credit Rating Table [Line Items]
Total investments		$ 2,639,345	$ 2,709,137
Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments		2,483,861	2,553,539
Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments		2,113,795	2,224,236
Term loans
Investments by Credit Rating Table [Line Items]
Total investments		875,560	1,061,934
Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments		548,101	372,473
Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments		217,459	285,609
Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments		288,252	336,171
Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments		31,182	32,456
Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments		151,124	133,409
Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments		2,117	2,184
Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments		370,066	329,303
Other investments
Investments by Credit Rating Table [Line Items]
Total investments		34,142	30,461
Equities
Investments by Credit Rating Table [Line Items]
Total investments		121,342	125,137
AAA
Investments by Credit Rating Table [Line Items]
Total investments	[1]	40,723	28,924
AAA | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	40,723	28,924
AAA | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	2,416	3,141
AAA | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
AAA | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
AAA | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
AAA | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	1,377	2,006
AAA | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
AAA | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
AAA | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	1,039	1,135
AAA | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	38,307	25,783
AA
Investments by Credit Rating Table [Line Items]
Total investments	[1]	580,625	591,612
AA | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	580,625	591,612
AA | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	385,803	454,770
AA | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
AA | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	16,032	36,128
AA | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	217,459	285,609
AA | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
AA | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	602	0
AA | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	151,124	132,460
AA | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	586	573
AA | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	194,822	136,842
A
Investments by Credit Rating Table [Line Items]
Total investments	[1]	179,728	147,059
A | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	179,728	147,059
A | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	118,848	112,156
A | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
A | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	90,087	81,401
A | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
A | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	23,475	29,179
A | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	4,794	1,100
A | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
A | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	492	476
A | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	60,880	34,903
BBB
Investments by Credit Rating Table [Line Items]
Total investments	[1]	394,344	404,813
BBB | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	394,344	404,813
BBB | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	320,470	289,658
BBB | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
BBB | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	96,231	41,103
BBB | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
BBB | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	207,617	223,956
BBB | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	16,622	23,650
BBB | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	949
BBB | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
BBB | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	73,874	115,155
BB
Investments by Credit Rating Table [Line Items]
Total investments	[1]	103,753	49,291
BB | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	103,753	49,291
BB | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	103,251	49,291
BB | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	23,218	9,617
BB | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	55,066	9,003
BB | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
BB | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	23,675	29,695
BB | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	1,292	976
BB | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
BB | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
BB | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	502	0
B
Investments by Credit Rating Table [Line Items]
Total investments	[1]	691,533	837,894
B | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	691,533	837,894
B | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	691,533	837,894
B | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	530,118	761,168
B | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	152,648	58,345
B | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
B | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	8,767	18,381
B | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
B | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
B | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
B | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CCC
Investments by Credit Rating Table [Line Items]
Total investments	[1]	362,864	359,881
CCC | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	362,864	359,881
CCC | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	362,864	351,522
CCC | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	247,478	215,909
CCC | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	113,723	135,613
CCC | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CCC | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	1,663	0
CCC | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CCC | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CCC | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CCC | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	8,359
CC
Investments by Credit Rating Table [Line Items]
Total investments	[1]	21,459	6,823
CC | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	21,459	6,823
CC | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	21,459	6,823
CC | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	15,191	6,823
CC | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	6,268	0
CC | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CC | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CC | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CC | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CC | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
CC | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
C
Investments by Credit Rating Table [Line Items]
Total investments	[1]	7,777	2,119
C | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	7,777	2,119
C | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	7,777	2,119
C | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	2,192	2,119
C | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	5,585	0
C | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
C | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
C | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
C | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
C | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
C | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
D
Investments by Credit Rating Table [Line Items]
Total investments	[1]	35,226	2,497
D | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	35,226	2,497
D | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	35,226	2,497
D | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	28,046	0
D | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	3,956	0
D | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
D | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
D | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	3,224	2,497
D | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
D | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
D | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
Not Rated
Investments by Credit Rating Table [Line Items]
Total investments	[1]	65,829	122,626
Not Rated | Fixed income and short term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	65,829	122,626
Not Rated | Fixed income instruments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	64,148	114,365
Not Rated | Term loans
Investments by Credit Rating Table [Line Items]
Total investments	[1]	29,317	66,298
Not Rated | Fixed maturities | Corporate bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	8,505	10,880
Not Rated | Fixed maturities | U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
Not Rated | Fixed maturities | Asset-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	21,678	32,954
Not Rated | Fixed maturities | Mortgage-backed securities
Investments by Credit Rating Table [Line Items]
Total investments	[1]	4,648	4,233
Not Rated | Fixed maturities | Non-U.S. government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
Not Rated | Fixed maturities | Municipal government and government agency bonds
Investments by Credit Rating Table [Line Items]
Total investments	[1]	0	0
Not Rated | Short-term investments
Investments by Credit Rating Table [Line Items]
Total investments	[1]	$ 1,681	$ 8,261

[1] For individual fixed maturity investments, Standard & Poor’s ratings are used. In the absence of a Standard & Poor’s rating, ratings from Moody’s are used, followed by ratings from Fitch, followed by ratings from KBRA, followed by ratings from DBRS.