Investment - Summary of Ranges and Weighted Averages of Significant Unobservable Inputs Used to Quantitative Assessments (Detail) - Advisor Engine [Member] - Preferred Stock [Member] - Level 3 [Member]	6 Months Ended
	Jun. 30, 2019
Minimum [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs Assets Quantitative Informations [Line Items]
Revenue multiple	4.7
Maximum [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs Assets Quantitative Informations [Line Items]
Revenue multiple	5.4
Weighted Average [Member] | Market Approach Valuation Technique [Member]
Fair Value Inputs Assets Quantitative Informations [Line Items]
Revenue multiple	5.0
Weighted Average [Member] | Income Approach Valuation Technique [Member]
Fair Value Inputs Assets Quantitative Informations [Line Items]
Weighted average cost of capital ("WACC")	26.00%