Net loss per share - Computation of Net Loss per Share on Basic and Diluted Basis (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income / loss (numerator):
Net income / (loss) from continuing operations	$ 137,000	$ (1,566,000)	$ (6,742,000)	$ (1,730,000)
Income from discontinued operations	986,000		1,008,000
Noncontrolling interest	4,664,000		5,559,000
Conversion of preferred units	10,456,000		10,456,000
Net loss attributable to common shareholders	$ (13,997,000)	$ (1,566,000)	$ (21,749,000)	$ (1,730,000)
Weighted-average shares (denominator):
Total weighted-average shares	95,971,706	3,301,667	72,234,717	3,301,667
Net loss per share- basic and diluted:
Loss from continuing operations	$ (0.16)	$ (0.47)	$ (0.31)	$ (0.52)
Income from discontinued operations	$ 0.01		$ 0.01
Net loss per share- basic and diluted	$ (0.15)	$ (0.47)	$ (0.30)	$ (0.52)
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		28,134,496
2,770 Property Contribution [Member] | Class B Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	634,408		431,117
Alaska Joint Venture Acquisition [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	9,584,482		4,818,718
Subscription Agreement [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	363,115		182,561
Board of Trustees [Member] | Class A Common Shares [Member]
Weighted-average shares (denominator):
Total weighted-average shares	6,286		3,160