Credit Facility and Convertible Debentures - Credit Arrangements Narrative (Detail) - USD ($)		1 Months Ended			3 Months Ended		6 Months Ended
	May 15, 2018	Jul. 26, 2018	May 31, 2018	May 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Apr. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Proceeds from credit facility							$ 35,000,000	$ 50,000,000
Credit facility repayment feature liability					$ 2,979,000		2,979,000			$ 0
Debt Instrument, Gain (Loss) from Covenant Liability					(2,500,000)
Interest expense					3,422,000	$ 2,112,000	6,112,000	3,310,000
Subsequent Event [Member]
Debt Instrument [Line Items]
Proceeds from sale of equity method investments		$ 55,000,000
Convertible Debt
Debt Instrument [Line Items]
Interest expense					$ 400,000	1,200,000	$ 1,300,000	2,300,000
Debt instrument, interest rate, effective percentage					8.70%		8.70%
Interest paid					$ 1,100,000	1,500,000	$ 1,100,000	1,500,000
Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Debt Covenant, Cash and Cash Equivalent Threshold, Interest and Prepayment Fee Percent			100.00%
Amended Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 100,000,000.0
Long-term line of credit outstanding					85,000,000		85,000,000
Debt Instrument, Debt Covenants, Cash and Cash Equivalent Threshold			50,000,000
Liquidity threshold			20,000,000
Minimum aggregate appraised value plus liquidity threshold			350,000,000
Debt Instrument, Debt Covenant, Deployment Threshold			40,000,000.0
Debt Instrument, Debt Covenant, Expense Threshold			11,500,000.0
Unamortized discount and debt issuance costs					6,000,000		6,000,000
Long-term debt					79,000,000		79,000,000
Credit facility repayment feature liability			500,000		3,000,000		3,000,000
2018 Debentures [Member]
Debt Instrument [Line Items]
Repayments of debt	$ 41,000,000
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					$ 2,900,000	$ 900,000	$ 4,700,000	$ 900,000
Debt instrument, interest rate, effective percentage					14.90%		14.90%
Interest paid					$ 1,300,000		$ 2,600,000
Revolving Credit Facility | Credit Facility [Member]
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%
Revolving Credit Facility | Credit Facility [Member] | Interest Rate Application A | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate				8.50%
Revolving Credit Facility | Credit Facility [Member] | Interest Rate Application A | London Interbank Offered Rate (LIBOR) | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate				1.00%
Revolving Credit Facility | Credit Facility [Member] | Interest Rate Application B
Debt Instrument [Line Items]
Basis spread on variable rate				2.00%
Revolving Credit Facility | Credit Facility [Member] | Interest Rate Application B | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate				7.50%
Revolving Credit Facility | Credit Facility [Member] | Interest Rate Application B | London Interbank Offered Rate (LIBOR)
Debt Instrument [Line Items]
Basis spread on variable rate				1.00%
Revolving Credit Facility | Credit Facility [Member] | Interest Rate Application B | Federal Funds Effective Swap Rate
Debt Instrument [Line Items]
Basis spread on variable rate				0.50%
Revolving Credit Facility | Amended Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			15,000,000.0
Term Loan [Member] | Amended Credit Facility [Member]
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