Net loss per share - Computation of Net Loss per Share on Basic and Diluted Basis (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income / loss (numerator):
Loss from continuing operations	$ (3,861,000)	$ (2,675,000)	$ (10,603,000)	$ (4,405,000)
Income from discontinued operations			1,008,000
Noncontrolling interest	3,798,000		9,357,000
Conversion of preferred units			10,456,000
Net loss attributable to common shareholders	$ (7,659,000)	$ (2,675,000)	$ (29,408,000)	$ (4,405,000)
Weighted-average shares (denominator):
Total weighted-average shares	162,725,150	3,301,667	102,729,661	3,301,667
Net loss per share- basic and diluted:
Loss from continuing operations	$ (0.05)	$ (0.81)	$ (0.30)	$ (1.33)
Income from discontinued operations			$ 0.01
Net loss per share- basic and diluted	$ (0.05)	$ (0.81)	$ (0.29)	$ (1.33)
Formation Transactions [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	3,301,000	3,301,000	3,301,000	3,301,000
Formation Transactions [Member] | Class B Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	667	667	667	667
2012 Offering [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	35,362,998		35,362,998
2013 Offering [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	46,718,750		34,397,321
2,770 Property Contribution [Member] | Class B Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	634,408		499,625
IPO [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	26,854,220		9,049,774
2013 Concurrent Private Placements [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	2,853,261		961,538
IPO Over-allotment Exercise [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	2,949,169		993,860
Alaska Joint Venture Acquisition [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	43,609,394		17,891,033
Subscription Agreement [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	434,783		267,559
Board of Trustees [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	6,500		4,286