Stockholders' Equity - Summary of Changes in Stockholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Shareholders Equity [Line Items]
Beginning balance, value	$ 584,511	$ 566,669	$ 581,164	$ 553,695
Net earnings	11,890	10,944	36,015	34,516
Dividends declared	(14,662)	(13,145)	(43,845)	(39,060)
Shares issued pursuant to ATM Program, net, value	2,154	6,938	8,817	20,652
Shares issued pursuant to dividend reinvestment, value	364	325	$ 1,082	$ 1,080
Shares issued pursuant to dividend reinvestment, shares			35,424	40,323
Stock-based compensation/settlements, value	664	461	$ 1,688	$ 1,309
Ending balance, value	584,921	572,192	584,921	572,192
Common Stock [Member]
Shareholders Equity [Line Items]
Beginning balance, value	$ 411	$ 403	$ 409	$ 397
Beginning balance, shares	41,108,000	40,262,000	40,855,000	39,696,000
Shares issued pursuant to ATM Program, net, value	$ 1	$ 2	$ 3	$ 8
Shares issued pursuant to ATM Program, net, shares	70,000	248,000	283,000	785,000
Shares issued pursuant to dividend reinvestment, shares	12,000	12,000	36,000	41,000
Stock-based compensation/settlements, shares			16,000
Ending balance, value	$ 412	$ 405	$ 412	$ 405
Ending balance, share	41,190,000	40,522,000	41,190,000	40,522,000
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value	$ 646,581	$ 620,183	$ 638,178	$ 604,872
Shares issued pursuant to ATM Program, net, value	2,153	6,936	8,814	20,644
Shares issued pursuant to dividend reinvestment, value	364	325	1,082	1,080
Stock-based compensation/settlements, value	664	461	1,688	1,309
Ending balance, value	649,762	627,905	649,762	627,905
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value	(62,481)	(53,917)	(57,423)	(51,574)
Net earnings	11,890	10,944	36,015	34,516
Dividends declared	(14,662)	(13,145)	(43,845)	(39,060)
Ending balance, value	$ (65,253)	$ (56,118)	$ (65,253)	$ (56,118)