Investment in Securities - Equity Securities by Market Sector Distribution (Detail) - Credit Concentration Risk - Equity Securities	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
Consumer goods
Concentration Risk [Line Items]
Distribution, percentage	19.70%	18.90%
Energy and utilities
Concentration Risk [Line Items]
Distribution, percentage	6.30%	8.00%
Finance
Concentration Risk [Line Items]
Distribution, percentage	15.70%	18.00%
Healthcare
Concentration Risk [Line Items]
Distribution, percentage	14.20%	13.00%
Industrials
Concentration Risk [Line Items]
Distribution, percentage	7.10%	7.60%
Information technology
Concentration Risk [Line Items]
Distribution, percentage	27.20%	25.00%
Other
Concentration Risk [Line Items]
Distribution, percentage	9.80%	9.50%