Stockholders' Equity - Summary of Changes in Stockholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Shareholders Equity [Line Items]
Beginning balance, value	$ 586,475	$ 578,235	$ 589,439	$ 581,164
Net earnings	10,973	13,198	23,673	24,125
Dividends declared	(15,768)	(14,628)	(31,400)	(29,183)
Shares issued pursuant to dividend reinvestment, value	16	361	$ 410	$ 718
Shares issued pursuant to dividend reinvestment, shares			13,027	23,571
Stock-based compensation/settlements, value	868	665	$ 1,356	$ 1,024
Ending balance, value	594,508	584,511	594,508	584,511
ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to ATM Program, net, value	11,944	6,680	11,916	6,663
Cumulative Effect, Period of Adoption, Adjustment [Member]
Shareholders Equity [Line Items]
Beginning balance, value			(886)
Common Stock [Member]
Shareholders Equity [Line Items]
Beginning balance, value	$ 414	$ 409	$ 414	$ 409
Beginning balance, shares	41,391,000	40,883,000	41,368,000	40,855,000
Shares issued pursuant to dividend reinvestment, shares	1,000	12,000	13,000	24,000
Stock-based compensation/settlements, shares			11,000	16,000
Ending balance, value	$ 418	$ 411	$ 418	$ 411
Ending balance, share	41,796,000	41,108,000	41,796,000	41,108,000
Common Stock [Member] | ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to ATM Program, net, value	$ 4	$ 2	$ 4	$ 2
Shares issued pursuant to ATM Program, net, shares	404,000	213,000	404,000	213,000
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value	$ 656,981	$ 638,877	$ 656,127	$ 638,178
Shares issued pursuant to dividend reinvestment, value	16	361	410	718
Stock-based compensation/settlements, value	868	665	1,356	1,024
Ending balance, value	669,805	646,581	669,805	646,581
Additional Paid-in-Capital [Member] | ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to ATM Program, net, value	11,940	6,678	11,912	6,661
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value	(70,920)	(61,051)	(67,102)	(57,423)
Net earnings	10,973	13,198	23,673	24,125
Dividends declared	(15,768)	(14,628)	(31,400)	(29,183)
Ending balance, value	$ (75,715)	$ (62,481)	(75,715)	$ (62,481)
Dividends Paid in Excess of Earnings [Member] | Cumulative Effect, Period of Adoption, Adjustment [Member]
Shareholders Equity [Line Items]
Beginning balance, value			$ (886)