Shareholders' Equity - Summary of Changes in Shareholders' Equity (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Shareholders Equity [Line Items]
Beginning balance, value			$ 553,695,000
Net earnings	$ 13,540,000	$ 15,106,000	23,572,000	$ 24,810,000
Dividends declared — $0.64 per share			(25,915,000)
Shares issued pursuant to ATM Program, net			13,714,000
Shares issued pursuant to dividend reinvestment, value			$ 755,000	$ 628,000
Shares issued pursuant to dividend reinvestment, shares			28,884	24,284
Stock-based compensation, value			$ 848,000
Ending balance, value	566,669,000		566,669,000
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			$ 6,000
Shares issued during period, shares			537,000
Shares issued pursuant to dividend reinvestment, shares			29,000
Ending balance, value	$ 403,000		$ 403,000
Ending balance, share	40,262,000		40,262,000
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value			$ 604,872,000
Shares issued pursuant to ATM Program, net			13,708,000
Shares issued pursuant to dividend reinvestment, value			755,000
Stock-based compensation, value			848,000
Ending balance, value	$ 620,183,000		620,183,000
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value			(51,574,000)
Net earnings			23,572,000
Dividends declared — $0.64 per share			(25,915,000)
Ending balance, value	$ (53,917,000)		$ (53,917,000)