Noncash transactions - Additional Information (Detail) (USD $)										1 Months Ended			1 Months Ended
	Jun. 14, 2013	Feb. 28, 2013	Jun. 11, 2013 Alaska Joint Venture Acquisition [Member] Operating Partnership [Member]	Feb. 28, 2013 Series C convertible units [Member] Operating Partnership [Member]	Sep. 30, 2013 Class B Common Shares [Member]	Feb. 28, 2013 Class B Common Shares [Member]	Dec. 31, 2012 Class B Common Shares [Member]	Jun. 10, 2013 Series D Convertible Units [Member] Operating Partnership [Member]	Jun. 10, 2013 Series E Convertible Units [Member] Operating Partnership [Member]	Jun. 14, 2013 Class A Common Shares [Member]	Sep. 30, 2013 Class A Common Shares [Member]	Dec. 31, 2012 Class A Common Shares [Member]	Jun. 14, 2013 Class A Common Shares [Member] Operating Partnership [Member]	Jun. 11, 2013 Class A Common Shares [Member] Alaska Joint Venture Acquisition [Member]	Jun. 11, 2013 Class A Units [Member] Alaska Joint Venture Acquisition [Member] Operating Partnership [Member]
Nonmonetary Transaction [Line Items]
Net carrying cost		$ 386,500,000
Units outstanding				31,085,974
Common shares issued					635,075	634,408	667				184,856,219	38,663,998
Issuance of units			12,395,965					4,375,000	4,375,000						12,395,965
Acquisition of Joint Venture														43,609,394
Preferred Units issued	653,492
Conversion to Class A units										653,492			653,492
Additional units issued										705,167