Shareholders' equity - Additional Information 2 (Detail) (USD $)	3 Months Ended	9 Months Ended		9 Months Ended	12 Months Ended	3 Months Ended	9 Months Ended	12 Months Ended		1 Months Ended		1 Months Ended	9 Months Ended		9 Months Ended	12 Months Ended	1 Months Ended	3 Months Ended	9 Months Ended		3 Months Ended	9 Months Ended				3 Months Ended	9 Months Ended			3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Jun. 14, 2013	Sep. 30, 2013 2012 Equity Incentive Plan [Member]	Dec. 31, 2012 2012 Equity Incentive Plan [Member]	Sep. 30, 2013 Consolidated Subsidiaries [Member]	Sep. 30, 2013 Consolidated Subsidiaries [Member]	Dec. 31, 2012 3.5% Convertible Perpetual Preferred Units [Member] Operating Partnership [Member]	Jun. 14, 2013 3.5% Convertible Perpetual Preferred Units [Member] Operating Partnership [Member]	Jun. 14, 2013 Class A Common Shares [Member]	Dec. 31, 2012 Class A Common Shares [Member]	Apr. 04, 2013 Class A Common Shares [Member] 2012 Equity Incentive Plan [Member]	Sep. 30, 2013 Class A Common Shares [Member] 2012 Equity Incentive Plan [Member]	Dec. 31, 2012 Class A Common Shares [Member] 2012 Equity Incentive Plan [Member]	Sep. 30, 2013 Class A Common Shares [Member] Subscription Agreement [Member]	Dec. 31, 2012 Class A Common Shares [Member] Subscription Agreement [Member] Option One [Member]	Jun. 14, 2013 Class A Common Shares [Member] Operating Partnership [Member]	Sep. 30, 2013 Class A Common Shares [Member] Equity Option [Member] 2012 Equity Incentive Plan [Member]	Sep. 30, 2013 Class A Common Shares [Member] Equity Option [Member] 2012 Equity Incentive Plan [Member]	Sep. 30, 2013 Class A Common Shares [Member] Equity Option [Member] Board of Trustees [Member] 2012 Equity Incentive Plan [Member]	Sep. 30, 2013 Class A Units [Member]	Sep. 30, 2013 Class A Units [Member]	Dec. 31, 2012 Class A Units [Member]	Sep. 30, 2013 Class A Units [Member] Operating Partnership [Member]	Dec. 31, 2012 Class A Units [Member] Operating Partnership [Member]	Sep. 30, 2013 Series C Convertible Units [Member]	Sep. 30, 2013 Series C Convertible Units [Member]	Sep. 30, 2013 Series C Convertible Units [Member] Operating Partnership [Member]	Sep. 30, 2013 Series D Convertible Units [Member] Operating Partnership [Member]	Sep. 30, 2013 Preferred Units [Member]	Sep. 30, 2013 Preferred Units [Member]	Apr. 30, 2013 Maximum [Member] Class A Common Shares [Member] 2012 Equity Incentive Plan [Member]	Apr. 30, 2013 Minimum [Member] Class A Common Shares [Member] 2012 Equity Incentive Plan [Member]
Changes In Equity And Comprehensive Income Line Items [Line Items]
Preferred Units issued			653,492					653,492
Rate of Preferred Units								3.50%
Annual distribution								$ 0.525
Fair market value per Preferred Unit								15.00
Preferred units outstanding									653,492
Issuance of shares under the plan											650,000			1,500,000																		6,000,000	1,500,000
Plan termination date				November 2022
Stock options granted					50,000
Vesting Period				4 years	4 years
Expiration period of options				10 years	10 years
Stock options cancelled				60,000
Reclassified grants to employees, stock options													485,000
Share-based compensation expense	$ 153,000	$ 606,000																$ 153,000	$ 494,000	$ 112,000
Common stock issued												6,500
Issuance of common shares															434,783	3,333,334
Aggregate purchase price of shares																50,000,000
Purchase price per share															$ 17.25	$ 15.00
Percentage of interest held																					6.90%	6.90%	0.10%
Units outstanding																								199,278,586	38,697,333			31,085,974	4,375,000
Preferred Units converted into Class A units										653,492							653,492
Noncontrolling interest	$ 3,798,000	$ 9,357,000				$ 230,000	$ 228,000														$ 670,000	$ 780,000				$ 4,698,000	$ 10,208,000			$ 0	$ 157,000