Equity - Additional Information (Detail) - USD ($)							3 Months Ended									12 Months Ended
	Nov. 18, 2016	Mar. 28, 2016	Aug. 18, 2015	Dec. 10, 2014	Oct. 31, 2014	Oct. 17, 2014	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, 2014	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Class of Stock [Line Items]
Issuance of common stock, net (in shares)	6,330,000	9,780,000	16,330,000
Proceeds from the issuance of common stock, net	$ 80,900,000	$ 105,800,000	$ 163,000,000													$ 200,402,000	$ 162,963,000	$ 0
Share price to the public (in usd per share)	$ 13.35	$ 11.35	$ 10.50
Dividends payable							$ 11,075,000				$ 7,704,000					$ 11,075,000	$ 7,704,000
Dividend, payable date																Jan. 15, 2015
Dividend, record date																Dec. 31, 2014
Dividends declared per common share (in usd per share)							$ 0.17	$ 0.17	$ 0.17	$ 0.17	$ 0.16	$ 0.16	$ 0.16	$ 0.16	$ 0.125	$ 0.68	$ 0.64	$ 0.125
Special Dividend [Member]
Class of Stock [Line Items]
Issuance of common stock, net (in shares)				8,974,249
Dividend, declared date						Oct. 17, 2014										Oct. 17, 2014
Dividends payable						$ 132,000,000
Dividend payable per share (in usd per share)						$ 5.88
Dividend, payable date				Dec. 10, 2014												Dec. 10, 2014
Dividend, record date					Oct. 31, 2014											Oct. 31, 2014
Cash portion of dividend				$ 33,000,000
Stock portion of dividend				$ 99,000,000
Over-Allotment Option [Member]
Class of Stock [Line Items]
Issuance of common stock, net (in shares)	830,000	1,280,000	2,130,000
At-The-Market Offering [Member]
Class of Stock [Line Items]
Issuance of common stock, net (in shares)																900,000.0
Proceeds from the issuance of common stock, net																$ 14,100,000
Aggregate offering price							$ 125,000,000.0									125,000,000.0
Common stock offering, remaining and available for issue							$ 111,100,000									$ 111,100,000
At-The-Market Offering [Member] | Weighted Average [Member]
Class of Stock [Line Items]
Share price to the public (in usd per share)							$ 15.31									$ 15.31