Goodwill and Intangible Assets - Summary of Gross Carrying Value, Accumulated Amortization and Weighted Average Amortization Period of Intangible Assets by Type (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($)
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount	$ 6,365
Finite-Lived Assets, Accumulated Amortization	(963)
Indefinite-Lived Assets, Gross Carrying Amount	4,535
ASCO Customer Relationships
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount	3,260
Finite-Lived Assets, Accumulated Amortization	$ (245)
Finite-Lived Assets, Weighted Average Amortization Period	10 years
ASCO VOBA
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount	$ 1,649
Finite-Lived Assets, Accumulated Amortization	$ (309)
Finite-Lived Assets, Weighted Average Amortization Period	4 years
BDM Broker Networks
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount	$ 976
Finite-Lived Assets, Accumulated Amortization	$ (49)
Finite-Lived Assets, Weighted Average Amortization Period	15 years
BDM Trade Name
Intangible Assets [Line Items]
Finite-Lived Assets, Gross Carrying Amount	$ 480
Finite-Lived Assets, Accumulated Amortization	$ (360)
Finite-Lived Assets, Weighted Average Amortization Period	1 year
ASCO European Licenses
Intangible Assets [Line Items]
Indefinite-Lived Assets, Gross Carrying Amount	$ 2,399
NUAL Lloyd's Syndicate Capacity
Intangible Assets [Line Items]
Indefinite-Lived Assets, Gross Carrying Amount	$ 2,136