Stockholders' Equity - Summary of Changes in Stockholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shareholders Equity [Line Items]
Beginning balance, value				$ 581,164				$ 553,695	$ 581,164	$ 553,695	$ 430,918
Net earnings	$ 13,708	$ 11,890	$ 13,198	10,927	$ 13,190	$ 10,944	$ 13,540	10,032	49,723	47,706	47,186
Dividends declared									(59,402)	(53,555)	(43,675)
Shares issued pursuant to dividend reinvestment, value									$ 1,451	$ 1,403	1,271
Shares issued pursuant to dividend reinvestment, shares									46,896	51,920
Stock-based compensation and settlements, value									$ 2,353	$ 1,777	155
Ending balance, value	589,439				581,164				589,439	581,164	553,695
Equity Offering [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value											104,312
ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value									14,150	30,138	13,528
Common Stock [Member]
Shareholders Equity [Line Items]
Beginning balance, value				$ 409				$ 397	$ 409	$ 397	$ 344
Beginning balance, shares				40,855,000				39,696,000	40,855,000	39,696,000	34,393,000
Shares issued pursuant to dividend reinvestment, value									$ 1	$ 1	$ 1
Shares issued pursuant to dividend reinvestment, shares									47,000	52,000	48,000
Stock-based compensation and settlements, shares									17,000	1,000	27,000
Ending balance, value	$ 414				$ 409				$ 414	$ 409	$ 397
Ending balance, share	41,368,000				40,855,000				41,368,000	40,855,000	39,696,000
Common Stock [Member] | Equity Offering [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value											$ 47
Shares issued pursuant to Offering/Program net, shares											4,715,000
Common Stock [Member] | ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value									$ 4	$ 11	$ 5
Shares issued pursuant to Offering/Program net, shares									449,000	1,106,000	513,000
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value				$ 638,178				$ 604,872	$ 638,178	$ 604,872	$ 485,659
Shares issued pursuant to dividend reinvestment, value									1,450	1,402	1,270
Stock-based compensation and settlements, value									2,353	1,777	155
Ending balance, value	$ 656,127				$ 638,178				656,127	638,178	604,872
Additional Paid-in-Capital [Member] | Equity Offering [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value											104,265
Additional Paid-in-Capital [Member] | ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value									14,146	30,127	13,523
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value				$ (57,423)				$ (51,574)	(57,423)	(51,574)	(55,085)
Net earnings									49,723	47,706	47,186
Dividends declared									(59,402)	(53,555)	(43,675)
Ending balance, value	$ (67,102)				$ (57,423)				$ (67,102)	$ (57,423)	$ (51,574)