Shareholders' Equity - Summary of Changes in Shareholders' Equity (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Shareholders Equity [Line Items]
BALANCE, DECEMBER 31, 2016, value			$ 430,918,000
Net earnings	$ 9,340,000	$ 8,804,000	34,150,000	$ 30,083,000
Dividends declared — $0.84 per share			(30,828,000)
Shares issued pursuant to Equity Offering, net, value			104,312,000
Shares issued pursuant to ATM Program, net, value			8,545,000
Shares issued pursuant to dividend reinvestment, value			$ 947,000
Shares issued pursuant to dividend reinvestment, shares			36,865
Stock-based compensation, value			$ (200,000)
BALANCE, JUNE 30, 2017, value	547,844,000		547,844,000
Common Stock [Member]
Shareholders Equity [Line Items]
BALANCE, DECEMBER 31, 2016, value			$ 344,000
BALANCE, DECEMBER 31, 2016, shares			34,393,000
Shares issued pursuant to Equity Offering, net, value			$ 47,000
Shares issued pursuant to Equity Offering, net, shares			4,715,000
Shares issued pursuant to ATM Program, net, value			$ 3,000
Shares issued pursuant to ATM Program, net, shares			329,000
Shares issued pursuant to dividend reinvestment, value			$ 1,000
Shares issued pursuant to dividend reinvestment, shares			37,000
Stock-based compensation, shares			27,000
BALANCE, JUNE 30, 2017, value	$ 395,000		$ 395,000
BALANCE, DECEMBER 31, 2016, shares	39,501,000		39,501,000
Additional Paid-in-Capital [Member]
Shareholders Equity [Line Items]
BALANCE, DECEMBER 31, 2016, value			$ 485,659,000
Shares issued pursuant to Equity Offering, net, value			104,265,000
Shares issued pursuant to ATM Program, net, value			8,542,000
Shares issued pursuant to dividend reinvestment, value			946,000
Stock-based compensation, value			(200,000)
BALANCE, JUNE 30, 2017, value	$ 599,212,000		599,212,000
Dividends Paid in Excess of Earnings [Member]
Shareholders Equity [Line Items]
BALANCE, DECEMBER 31, 2016, value			(55,085,000)
Net earnings			34,150,000
Dividends declared — $0.84 per share			(30,828,000)
BALANCE, JUNE 30, 2017, value	$ (51,763,000)		$ (51,763,000)