Investments in Securities - Equity Securities by Market Sector Distribution (Detail) - Equity Securities [Member] - Credit Concentration Risk [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Concentration Risk [Line Items]
Equity securities with market sector distribution	100.00%	100.00%
Consumer Goods [Member]
Concentration Risk [Line Items]
Equity securities with market sector distribution	21.40%	20.40%
Energy and Utilities [Member]
Concentration Risk [Line Items]
Equity securities with market sector distribution	10.80%	13.30%
Financials [Member]
Concentration Risk [Line Items]
Equity securities with market sector distribution	19.10%	19.10%
Healthcare [Member]
Concentration Risk [Line Items]
Equity securities with market sector distribution	14.90%	14.00%
Industrials [Member]
Concentration Risk [Line Items]
Equity securities with market sector distribution	8.10%	8.40%
Information Technology [Member]
Concentration Risk [Line Items]
Equity securities with market sector distribution	17.30%	16.20%
Other [Member]
Concentration Risk [Line Items]
Equity securities with market sector distribution	8.40%	8.60%