Fair Value of Assets and Liabilities - Summary of Valuation Techniques and Significant Unobservable Inputs Used in Recurring Level 3 Fair Value Measurements of Assets and Liabilities (Detail) - Equity Investments [Member] - USD ($)	6 Months Ended	12 Months Ended
	Jun. 30, 2019	Dec. 31, 2018
Precedent Market Transactions [Member] | Equity Method Offering Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value	$ 8.73	$ 8.73
Precedent Arms Length Offer [Member] | Financial Condition and Operating Performance Enterprise Value [Member] | Minimum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value	$ 6,014,000	$ 6,014,000
Precedent Arms Length Offer [Member] | Financial Condition and Operating Performance Enterprise Value [Member] | Maximum [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity Value	7,214,000	7,214,000
Level 3 [Member] | Investee Financial Analysis [Member] | Measurement Input Financial Condition and Operational Performance [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity investments	6,314,000	5,683,000
Level 3 [Member] | Investee Book Value Adjusted for Market Appreciation [Member] | Financial Condition and Operating Performance of the Investee [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity investments	2,028,000	1,850,000
Level 3 [Member] | Precedent Market Transactions [Member] | Equity Method Offering Price [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity investments	$ 1,455,000	1,455,000
Level 3 [Member] | Investee Book Value [Member] | Equity Method Valuation Indicated By Investee Filings [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Equity investments		$ 209,000