Investment in Securities - Equity Securities by Market Sector Distribution (Detail) - Equity Securities - Credit Concentration Risk	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
Consumer Goods
Concentration Risk [Line Items]
Distribution, percentage	18.90%	21.10%
Energy and Utilities
Concentration Risk [Line Items]
Distribution, percentage	8.00%	8.20%
Finance
Concentration Risk [Line Items]
Distribution, percentage	18.00%	18.10%
Healthcare
Concentration Risk [Line Items]
Distribution, percentage	13.00%	13.50%
Industrials
Concentration Risk [Line Items]
Distribution, percentage	7.60%	9.00%
Information Technology
Concentration Risk [Line Items]
Distribution, percentage	25.00%	22.60%
Other
Concentration Risk [Line Items]
Distribution, percentage	9.50%	7.50%