Commitments and Contingencies - Stock for Redemptions Based on Number of Years Stock Held (Detail)	6 Months Ended
	Jun. 30, 2018
Less than 1 [Member]
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Redemption price	0.00%
More than 1 but less than 2 [Member]
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Redemption price	90.00%
More than 2 but less than 3 [Member]
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Redemption price	92.50%
More than 3 but less than 4 [Member]
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Redemption price	95.00%
More than 4 [Member]
Financial Instruments Subject to Mandatory Redemption by Settlement Terms [Line Items]
Redemption price	100.00%