Shareholders' Equity - Summary of Changes in Shareholders' Equity (Detail) - USD ($)		3 Months Ended								12 Months Ended
	Nov. 25, 2015	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Shareholders Equity [Line Items]
Beginning balance, value					$ 406,561,000				$ 407,024,000	$ 406,561,000	$ 407,024,000	$ 415,091,000
Net earnings		$ 8,328,000	$ 8,804,000	$ 13,576,000	7,703,000	$ 19,893,000	$ 7,035,000	$ 11,619,000	(1,137,000)	38,411,000	37,410,000	23,418,000
Dividends										(35,385,000)	(38,897,000)	(32,402,000)
Shares issued pursuant to ATM Program, net, value										14,886,000
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										$ 897,000
Shares issued pursuant to dividend reinvestment, shares										42,681
Stock-based compensation, Value										$ 1,136,000	1,024,000	917,000
Ending balance, value		430,918,000				406,561,000				430,918,000	406,561,000	407,024,000
Common Stock [Member]
Shareholders Equity [Line Items]
Beginning balance, value					$ 334,000				$ 334,000	$ 334,000	$ 334,000	$ 334,000
Beginning balance, shares					33,422				33,417	33,422	33,417	33,397
Shares issued pursuant to ATM Program, net, value										$ 7,000
Shares issued pursuant to ATM Program, net, shares										653,000
Shares issued pursuant to stock dividends, value										$ 3,000
Shares issued pursuant to stock dividends, shares										256,000
Shares issued pursuant to dividend reinvestment, shares										43,000
Stock-based compensation, Shares										19,000	5,000	20,000
Ending balance, value		$ 344,000				$ 334,000				$ 344,000	$ 334,000	$ 334,000
Ending balance, shares		34,393				33,422				34,393	33,422	33,417
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value					$ 464,338,000				$ 463,314,000	$ 464,338,000	$ 463,314,000	$ 462,397,000
Shares issued pursuant to ATM Program, net, value										14,879,000
Shares issued pursuant to stock dividends, value										4,409,000
Shares issued pursuant to dividend reinvestment, value										897,000
Stock-based compensation, Value										1,136,000	1,024,000	917,000
Ending balance, value		$ 485,659,000				$ 464,338,000				485,659,000	464,338,000	463,314,000
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value					$ (58,111,000)				$ (56,624,000)	(58,111,000)	(56,624,000)	(47,640,000)
Net earnings										38,411,000	37,410,000	23,418,000
Dividends										(35,385,000)	(38,897,000)	(32,402,000)
Ending balance, value		$ (55,085,000)				$ (58,111,000)				$ (55,085,000)	$ (58,111,000)	$ (56,624,000)