Regulatory Requirements (Detail) $ in Millions	6 Months Ended
	Jun. 30, 2021 USD ($)
Regulatory Assets [Line Items]
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	$ 687.9
Amount of capital in excess of aggregate regulatory requirements	375.4
Disposal Group, Held-for-sale, Not Discontinued Operations
Regulatory Assets [Line Items]
Net assets held by regulated subsidiaries	51.9
Amount of capital in excess of aggregate regulatory requirements	$ 43.2