Regulatory Requirements - Additional Information (Detail) $ in Millions	3 Months Ended
	Mar. 31, 2019 USD ($)
Regulatory Capital Requirements [Abstract]
Minimum period required for financial resources to cover operating costs	1 year
Minimum period required for cash or highly liquid securities to cover operating costs	6 months
Net assets held by regulated subsidiaries	$ 605.7
Amount of capital in excess of aggregate regulatory requirements	$ 375.2