Investment in Securities - Equity Securities by Market Sector Distribution (Detail) - Equity Securities - Credit Concentration Risk	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Distribution, percentage	100.00%	100.00%
Consumer goods
Concentration Risk [Line Items]
Distribution, percentage	20.90%	21.10%
Energy and utilities
Concentration Risk [Line Items]
Distribution, percentage	9.10%	8.20%
Finance
Concentration Risk [Line Items]
Distribution, percentage	18.10%	18.10%
Healthcare
Concentration Risk [Line Items]
Distribution, percentage	13.40%	13.50%
Industrials
Concentration Risk [Line Items]
Distribution, percentage	8.00%	9.00%
Information technology
Concentration Risk [Line Items]
Distribution, percentage	22.70%	22.60%
Other
Concentration Risk [Line Items]
Distribution, percentage	7.80%	7.50%