Transactions with related parties - Transactions with ACGL (Details)			3 Months Ended		6 Months Ended
		Aug. 01, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jul. 02, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2014 USD ($) executive
Related Party Transaction [Line Items]
Ceding fee expense			$ 3,800,000	$ 4,800,000	$ 7,800,000	$ 9,000,000
Consolidated statement of income (loss) items:
Gross premiums written			157,927,000	161,978,000	392,829,000	348,667,000
Gross premiums ceded			(52,071,000)	(42,608,000)	(100,273,000)	(83,910,000)
Net premiums earned			(131,535,000)	(151,318,000)	(271,574,000)	(297,412,000)
Loss and loss adjustment expenses			(104,786,000)	(111,416,000)	(215,462,000)	(222,266,000)
Acquisition expenses			(29,486,000)	(35,417,000)	(57,853,000)	(69,391,000)
Interest expense			2,911,000	0	5,823,000	0
Preference dividends			1,109,000	4,908,000	2,280,000	9,815,000
Consolidated balance sheet items:
Total investments			2,639,345,000		2,639,345,000			$ 2,709,137,000
Premiums receivable			258,178,000		258,178,000		$ 281,541,000	273,657,000
Prepaid reinsurance premiums			131,919,000		131,919,000			132,577,000
Deferred acquisition costs, net			64,149,000		64,149,000			64,044,000
Funds held by reinsurers			41,112,000		41,112,000			42,505,000
Reserve for losses and loss adjustment expenses			1,353,049,000	1,126,080,000	1,353,049,000	1,126,080,000		1,263,628,000		$ 1,032,760,000
Unearned premiums			456,170,000		456,170,000			438,907,000
Losses payable			58,292,000		58,292,000			61,314,000
Reinsurance balances payable			72,776,000		72,776,000			77,066,000
Senior notes			172,554,000		172,554,000		$ 172,554,000	172,418,000
Amounts due to affiliates			4,542,000		4,542,000			4,467,000
Contingently redeemable preference shares			$ 52,351,000	221,175,000	$ 52,351,000	221,175,000		52,305,000		$ 220,992,000
ACGL and Subsidiaries | Arch Capital Group Limited
Related Party Transaction [Line Items]
Amount of investment by related party											$ 100,000,000
Percentage of common equity owned by related party			12.60%		12.60%						11.00%
Related party - number of executives serving as directors | executive											2
Percentage of preference equity owned by related party			6.60%		6.60%
Face amount of senior notes									$ 35,000,000
Consolidated statement of income (loss) items:
Interest expense			$ 582,000	0	$ 1,164,000	0
Preference dividends			73,000	325,000	151,000	650,000
Consolidated balance sheet items:
Total investments			775,016,000		775,016,000			815,528,000
Premiums receivable			96,475,000		96,475,000			106,462,000
Reinsurance recoverable on unpaid and paid losses and loss adjustment expenses			101,356,000		101,356,000			79,597,000
Prepaid reinsurance premiums			71,578,000		71,578,000			75,249,000
Deferred acquisition costs, net			29,894,000		29,894,000			31,609,000
Funds held by reinsurers			29,629,000		29,629,000			29,867,000
Reserve for losses and loss adjustment expenses			680,444,000		680,444,000			693,861,000
Unearned premiums			139,632,000		139,632,000			143,852,000
Losses payable			47,812,000		47,812,000			39,619,000
Reinsurance balances payable			53,387,000		53,387,000			62,301,000
Senior notes			34,511,000		34,511,000			34,484,000
Amounts due to affiliates			4,542,000		4,542,000			4,467,000
Other liabilities - contingent commissions			4,033,000		4,033,000			5,516,000
Contingently redeemable preference shares			3,465,000		3,465,000			$ 3,462,000
Quota share business | ACGL and Subsidiaries | Arch Capital Group Limited
Consolidated statement of income (loss) items:
Gross premiums ceded			(17,174,000)	(15,844,000)	(37,002,000)	(32,834,000)
Net premiums earned			(20,261,000)	(14,411,000)	(40,049,000)	(27,351,000)
Loss and loss adjustment expenses			(17,558,000)	(15,784,000)	(33,373,000)	(24,921,000)
Acquisition expenses	[1]		(3,319,000)	(3,584,000)	(7,857,000)	(6,688,000)
Direct business | ACGL and Subsidiaries | Arch Capital Group Limited
Consolidated statement of income (loss) items:
Gross premiums written			30,371,000	38,890,000	111,719,000	110,895,000
Net premiums earned			(51,067,000)	(66,913,000)	(109,853,000)	(128,751,000)
Loss and loss adjustment expenses			(40,759,000)	(54,894,000)	(87,300,000)	(100,263,000)
Acquisition expenses	[2]		$ (13,153,000)	$ (20,890,000)	$ (27,006,000)	$ (40,805,000)
Senior notes
Related Party Transaction [Line Items]
Face amount of senior notes									$ 175,000,000
Stated interest rate (percent)			6.50%		6.50%				6.50%
Contingently redeemable preference share | Arch Capital Group Limited
Related Party Transaction [Line Items]
Repurchase of preference shares		$ 11,500,000

[1] Acquisition expenses relating to the ACGL outward quota share agreements referred to above.

[2] Acquisition expenses relating to the ACGL inward quota share agreements referred to above. For the three months ended June 30, 2020 and 2019, the Company incurred ceding fees to Arch, in aggregate, of $3.8 million and $4.8 million, respectively, under these inward retrocession agreements. For the six months ended June 30, 2020 and 2019, the Company incurred ceding fees to Arch, in aggregate, of $7.8 million and $9.0 million, respectively, under these inward retrocession agreements.