Investment in Securities - Equity Securities by Market Sector Distribution (Detail) - Equity Securities - Credit Concentration Risk	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
Consumer Goods
Concentration Risk [Line Items]
Distribution, percentage	21.10%	20.20%
Energy and Utilities
Concentration Risk [Line Items]
Distribution, percentage	8.20%	8.60%
Finance
Concentration Risk [Line Items]
Distribution, percentage	18.10%	21.90%
Healthcare
Concentration Risk [Line Items]
Distribution, percentage	13.50%	11.80%
Industrials
Concentration Risk [Line Items]
Distribution, percentage	9.00%	9.50%
Information Technology
Concentration Risk [Line Items]
Distribution, percentage	22.60%	20.00%
Other
Concentration Risk [Line Items]
Distribution, percentage	7.50%	8.00%