Goodwill and Intangible Assets - Summary of Gross Carrying Value and Weighted Average Amortization Period of Intangible Assets by Type (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2018
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount		$ 6,420
Finite-Lived Assets, Accumulated Amortization		(647)
Indefinite-Lived Assets, Gross Carrying Amount		4,555
ASCO Customer Relationships
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount		3,289
Finite-Lived Assets, Accumulated Amortization		$ (164)
Finite-Lived Assets, Weighted Average Amortization Period	15 years	10 years
ASCO VOBA
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount		$ 1,663
Finite-Lived Assets, Accumulated Amortization		$ (208)
Finite-Lived Assets, Weighted Average Amortization Period		4 years
BDM Broker Networks
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount		$ 984
Finite-Lived Assets, Accumulated Amortization		$ (33)
Finite-Lived Assets, Weighted Average Amortization Period		15 years
BDM Trade Name
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount		$ 484
Finite-Lived Assets, Accumulated Amortization		$ (242)
Finite-Lived Assets, Weighted Average Amortization Period		1 year
ASCO European Licenses
Intangible Assets [Line Items]
Indefinite-Lived Assets, Gross Carrying Amount		$ 2,419
NUAL Lloyd's Syndicate Capacity
Intangible Assets [Line Items]
Indefinite-Lived Assets, Gross Carrying Amount		$ 2,136