Related Party Transactions - AmTrust (Details) - USD ($)	1 Months Ended		3 Months Ended								12 Months Ended
	Dec. 31, 2019	Nov. 30, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Investment in common and preferred shares											$ 69,213,000	$ 155,440,000	$ 0
Other income												1,453,000	2,679,000
Net investment income			$ 79,376,000	$ 85,472,000	$ 77,732,000	$ 78,696,000	$ 68,453,000	$ 69,430,000	$ 66,469,000	$ 66,319,000	321,276,000	270,671,000	208,789,000
Net realized and unrealized gains (losses)			152,664,000	$ 148,185,000	$ 269,711,000	$ 460,791,000	(158,213,000)	$ (57,223,000)	$ (54,418,000)	$ (143,030,000)	1,031,351,000	(412,884,000)	$ 190,334,000
Evergreen Parent L.P. | Affiliated entity
Related Party Transaction [Line Items]
Investment in common and preferred shares	$ 25,900,000	$ 200,000,000.0
Ownership of equity interest		8.50%
Other income												3,300,000
AmTrust Financial Services, Inc. | Affiliated entity
Related Party Transaction [Line Items]
Indirect investment	$ 240,115,000		$ 240,115,000				$ 200,000,000				240,115,000	200,000,000
Net investment income											7,700,000	300,000
Net realized and unrealized gains (losses)											$ 10,100,000	$ 0
Evergreen Parent L.P. | Trident Pine | Affiliated entity
Related Party Transaction [Line Items]
Ownership of equity interest		21.80%
AmTrust Financial Services, Inc. | Affiliated entity
Related Party Transaction [Line Items]
Voting ordinary shares acquired, percent		45.00%