Credit Facility and Convertible Debentures - Credit Arrangements Narrative (Detail) - USD ($)			1 Months Ended			3 Months Ended		9 Months Ended
	Oct. 15, 2018	May 15, 2018	Jul. 26, 2018	May 31, 2018	May 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Apr. 30, 2018
Debt Instrument [Line Items]
Proceeds from credit facility								$ 35,000,000	$ 50,000,000
Gain (loss) from covenant liability						$ (2,600,000)		(5,100,000)
Proceeds from sale of equity method investments			$ 55,000,000
Interest expense						3,310,000	$ 2,643,000	9,422,000	5,953,000
Convertible Debt
Debt Instrument [Line Items]
Interest expense						0	900,000	1,300,000	3,200,000
Debt instrument, interest rate, effective percentage										8.70%
Interest paid						0	0	1,100,000	1,500,000
Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Liquidity threshold					$ 20,000,000
Minimum aggregate appraised value plus liquidity threshold					350,000,000
Interest expense						3,300,000	1,700,000	8,000,000	2,600,000
Debt instrument, interest rate, effective percentage										14.90%
Interest paid						2,400,000	$ 1,200,000	4,900,000	$ 1,200,000
Credit Facility
Debt Instrument [Line Items]
Interest and prepayment fee percent				100.00%
Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity					75,000,000.0
Long-term line of credit outstanding					50,000,000
Proceeds from credit facility					$ 44,300,000
Debt instrument, term (in years)					3 years
Interest payable amount outstanding threshold					$ 50,000,000
Commitment fee percentage					0.75%
Credit Facility | Revolving Credit Facility | Interest Rate Application A | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate					8.50%
Credit Facility | Revolving Credit Facility | Interest Rate Application A | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate					1.00%
Credit Facility | Revolving Credit Facility | Interest Rate Application B
Debt Instrument [Line Items]
Basis spread on variable rate					2.00%
Credit Facility | Revolving Credit Facility | Interest Rate Application B | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate					7.50%
Credit Facility | Revolving Credit Facility | Interest Rate Application B | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate					1.00%
Credit Facility | Revolving Credit Facility | Interest Rate Application B | Federal Funds Effective Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate					0.50%
Amended Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 100,000,000.0
Long-term line of credit outstanding						85,000,000		85,000,000
Cash and cash equivalent threshold				50,000,000		50,000,000		50,000,000
Liquidity threshold				20,000,000
Minimum aggregate appraised value plus liquidity threshold				350,000,000
Deployment threshold				40,000,000.0
Expense threshold				11,500,000.0
Unamortized discount and debt issuance costs						5,200,000		5,200,000
Long-term debt						79,800,000		79,800,000
Credit facility repayment feature				500,000		$ 5,600,000		$ 5,600,000		$ 2,600,000
Amended Credit Facility | Subsequent Event
Debt Instrument [Line Items]
Repayments of lines of credit	$ 16,400,000
Periodic payment, interest	$ 2,800,000
Amended Credit Facility | Revolving Credit Facility
Debt Instrument [Line Items]
Maximum borrowing capacity				15,000,000.0
Amended Credit Facility | Term Loan
Debt Instrument [Line Items]
Maximum borrowing capacity				85,000,000.0
Long-term line of credit outstanding				35,000,000							$ 50,000,000
Proceeds from credit facility				$ 32,700,000
2018 Debentures
Debt Instrument [Line Items]
Repayments of debt		$ 41,000,000