Credit Facility and Convertible Debentures - Credit Arrangements Narrative (Detail)	1 Months Ended	3 Months Ended
	May 31, 2017 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)
Debt Instrument [Line Items]
Interest expense		$ 2,690,000	$ 1,198,000
Revolving Credit Facility | Line of Credit
Debt Instrument [Line Items]
Maximum borrowing capacity	$ 75,000,000
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%
Liquidity threshold	$ 20,000,000
Unrestricted cash multiplier	1.75
Minimum aggregate appraised value plus liquidity threshold	$ 350,000,000
Long-term debt, gross		50,000,000
Unamortized discount and debt issuance costs		4,300,000
Long-term debt		45,700,000
Interest expense		1,800,000
Interest paid		$ 1,300,000
Debt instrument, interest rate, effective percentage		15.00%
Revolving Credit Facility | Line of Credit | Interest Rate Application B
Debt Instrument [Line Items]
Basis spread on variable rate	2.00%
Revolving Credit Facility | Line of Credit | Interest Rate Application B | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	7.50%
Revolving Credit Facility | Line of Credit | London Interbank Offered Rate (LIBOR) | Interest Rate Application A
Debt Instrument [Line Items]
Basis spread on variable rate	8.50%
Revolving Credit Facility | Line of Credit | London Interbank Offered Rate (LIBOR) | Interest Rate Application A | Minimum
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility | Line of Credit | London Interbank Offered Rate (LIBOR) | Interest Rate Application B
Debt Instrument [Line Items]
Basis spread on variable rate	1.00%
Revolving Credit Facility | Line of Credit | Federal Funds Effective Swap Rate | Interest Rate Application B
Debt Instrument [Line Items]
Basis spread on variable rate	0.50%