Shareholders' Equity - Summary of Changes in Shareholders' Equity (Detail) - USD ($)		3 Months Ended								12 Months Ended
	Nov. 25, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Shareholders Equity [Line Items]
Beginning balance, value					$ 430,918,000				$ 406,561,000	$ 430,918,000	$ 406,561,000	$ 407,024,000
Net earnings		$ 13,036,000	$ 9,340,000	$ 15,106,000	9,704,000	$ 8,328,000	$ 8,804,000	$ 13,576,000	7,703,000	47,186,000	38,411,000	37,410,000
Dividends declared — $1.15, $1.03 and $1.16 per share for the year ended December 31, 2015, 2016 and 2017										(43,675,000)	(35,385,000)	(38,897,000)
Shares issued pursuant to stock dividends, value											4,412,000
Shares issued pursuant to stock dividends, shares	255,747
Shares issued pursuant to dividend reinvestment, value										$ 1,271,000	$ 897,000
Shares issued pursuant to dividend reinvestment, shares										47,922	42,681
Stock-based compensation and settlements										$ 155,000	$ 1,136,000	1,024,000
Ending balance, value		553,695,000				430,918,000				553,695,000	430,918,000	406,561,000
Equity Offering [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value										104,312,000
ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value										13,528,000	14,886,000
Common Stock [Member]
Shareholders Equity [Line Items]
Beginning balance, value					$ 344,000				$ 334,000	$ 344,000	$ 334,000	$ 334,000
Beginning balance, shares					34,393,000				33,422,000	34,393,000	33,422,000	33,417,000
Shares issued pursuant to stock dividends, value											$ 3,000
Shares issued pursuant to stock dividends, shares											256,000
Shares issued pursuant to dividend reinvestment, value										$ 1,000
Shares issued pursuant to dividend reinvestment, shares										48,000	43,000
Stock-based compensation and settlements										$ 27,000	$ 19,000	$ 5,000
Ending balance, value		$ 397,000				$ 344,000				$ 397,000	$ 344,000	$ 334,000
Ending balance, share		39,696,000				34,393,000				39,696,000	34,393,000	33,422,000
Common Stock [Member] | Equity Offering [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value										$ 47,000
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										$ 5,000	$ 7,000
Shares issued pursuant to Offering/Program net, shares										513,000	653,000
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value					$ 485,659,000				$ 464,338,000	$ 485,659,000	$ 464,338,000	$ 463,314,000
Shares issued pursuant to stock dividends, value											4,409,000
Shares issued pursuant to dividend reinvestment, value										1,270,000	897,000
Stock-based compensation and settlements										155,000	1,136,000	1,024,000
Ending balance, value		$ 604,872,000				$ 485,659,000				604,872,000	485,659,000	464,338,000
Additional Paid-in-Capital [Member] | Equity Offering [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value										104,265,000
Additional Paid-in-Capital [Member] | ATM Program [Member]
Shareholders Equity [Line Items]
Shares issued pursuant to Offering/Program net, value										13,523,000	14,879,000
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value					$ (55,085,000)				$ (58,111,000)	(55,085,000)	(58,111,000)	(56,624,000)
Net earnings										47,186,000	38,411,000	37,410,000
Dividends declared — $1.15, $1.03 and $1.16 per share for the year ended December 31, 2015, 2016 and 2017										(43,675,000)	(35,385,000)	(38,897,000)
Ending balance, value		$ (51,574,000)				$ (55,085,000)				$ (51,574,000)	$ (55,085,000)	$ (58,111,000)