Shareholders' Equity - Summary of Changes in Shareholders' Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Shareholders Equity [Line Items]
Beginning balance, value			$ 406,561
Net earnings	$ 8,804	$ 7,035	30,083	$ 17,517
Dividends			(25,644)
Stock dividends, value			4,742
Shares issued pursuant to ATM Program, net, value			2,940
Stock-based compensation, Value			1,153
Ending balance, value	419,835		419,835
Common Stock [Member]
Shareholders Equity [Line Items]
Beginning balance, value			$ 334
Beginning balance, shares			33,422
Stock dividends, value			$ 3
Stock dividends, Shares			286
Shares issued pursuant to ATM Program, net, value			$ 2
Shares issued pursuant to ATM Program, net			145
Stock-based compensation, Shares			19
Ending balance, value	$ 339		$ 339
Ending balance, shares	33,872		33,872
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
Beginning balance, value			$ 464,338
Stock dividends, value			4,739
Shares issued pursuant to ATM Program, net, value			2,938
Stock-based compensation, Value			1,153
Ending balance, value	$ 473,168		473,168
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
Beginning balance, value			(58,111)
Net earnings			30,083
Dividends			(25,644)
Ending balance, value	$ (53,672)		$ (53,672)