Stockholders' Equity - Summary of Changes in Stockholders' Equity (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Shareholders Equity [Line Items]
Beginning balance, value			$ 553,695,000
Net earnings	$ 10,944,000	$ 9,340,000	34,516,000	$ 34,150,000
Dividends declared — $0.96 per share			(39,060,000)
Shares issued pursuant to ATM Program, net			20,652,000
Shares issued pursuant to dividend reinvestment, value			$ 1,080,000	$ 947,000
Shares issued pursuant to dividend reinvestment, shares			40,323	36,865
Stock-based compensation, value			$ 1,309,000
Ending balance, value	572,192,000		572,192,000
Common Stock [Member]
Shareholders Equity [Line Items]
Beginning balance, value			$ 397,000
Beginning balance, shares			39,696,000
Shares issued pursuant to ATM Program, net			$ 8,000
Shares issued during period, shares			785,000
Shares issued pursuant to dividend reinvestment, shares			41,000
Ending balance, value	$ 405,000		$ 405,000
Ending balance, share	40,522,000		40,522,000
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value			$ 604,872,000
Shares issued pursuant to ATM Program, net			20,644,000
Shares issued pursuant to dividend reinvestment, value			1,080,000
Stock-based compensation, value			1,309,000
Ending balance, value	$ 627,905,000		627,905,000
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value			(51,574,000)
Net earnings			34,516,000
Dividends declared — $0.96 per share			(39,060,000)
Ending balance, value	$ (56,118,000)		$ (56,118,000)