Goodwill and Intangible Assets - Summary of Gross Carrying Value, Accumulated Amortization and Net Carrying Value of Intangible Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	$ 42,000	$ 42,000
Accumulated Amortization	(27,370)	(25,113)
Total amortization	14,630	16,887	$ 20,487
Intangible assets with an indefinite life, Carrying Value	87,031	87,031	87,031
Total intangible assets	101,661	103,918	$ 107,518
Distribution channel
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	20,000	20,000
Accumulated Amortization	(8,111)	(6,776)
Total amortization	11,889	13,224
Technology
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	15,000	15,000
Accumulated Amortization	(15,000)	(14,778)
Total amortization	0	222
Brand
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Value	7,000	7,000
Accumulated Amortization	(4,259)	(3,559)
Total amortization	2,741	3,441
Atrium
Finite-Lived Intangible Assets [Line Items]
Total amortization	14,630	16,665
Intangible assets with an indefinite life, Carrying Value	63,131	63,131
Total intangible assets	77,761	79,796
Atrium | Distribution channel
Finite-Lived Intangible Assets [Line Items]
Total amortization	11,889	13,224
Atrium | Technology
Finite-Lived Intangible Assets [Line Items]
Total amortization	0	0
Atrium | Brand
Finite-Lived Intangible Assets [Line Items]
Total amortization	2,741	3,441
StarStone
Finite-Lived Intangible Assets [Line Items]
Total amortization	0	222
Intangible assets with an indefinite life, Carrying Value	23,900	23,900
Total intangible assets	23,900	24,122
StarStone | Distribution channel
Finite-Lived Intangible Assets [Line Items]
Total amortization	0	0
StarStone | Technology
Finite-Lived Intangible Assets [Line Items]
Total amortization	0	222
StarStone | Brand
Finite-Lived Intangible Assets [Line Items]
Total amortization	0	0
Lloyd’s syndicate capacity
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	37,031	37,031
Lloyd’s syndicate capacity | Atrium
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	33,031	33,031
Lloyd’s syndicate capacity | StarStone
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	4,000	4,000
Licenses
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	19,900	19,900
Licenses | Atrium
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	0	0
Licenses | StarStone
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	19,900	19,900
Management contract
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	30,100	30,100
Management contract | Atrium
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	30,100	30,100
Management contract | StarStone
Finite-Lived Intangible Assets [Line Items]
Intangible assets with an indefinite life, Carrying Value	$ 0	$ 0