Shareholders' equity (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended	6 Months Ended				3 Months Ended	6 Months Ended				3 Months Ended		6 Months Ended						3 Months Ended	6 Months Ended
	Jun. 30, 2013 3.5% Convertible Perpetual Preferred Units	Jun. 30, 2013 3.5% Convertible Perpetual Preferred Units	Jun. 14, 2013 3.5% Convertible Perpetual Preferred Units	Jun. 30, 2014 Sponsor	Dec. 31, 2013 Sponsor	Jun. 30, 2014 Operating Partnership Class A Units	Jun. 30, 2014 Operating Partnership Class A Units	Dec. 31, 2013 Operating Partnership Class A Units	Jun. 30, 2014 Operating Partnership Class A Units Sponsor	Dec. 31, 2013 Operating Partnership Class A Units Sponsor	Jun. 30, 2014 Operating Partnership Series C Convertible Units	Jun. 30, 2013 Operating Partnership Series C Convertible Units	Jun. 30, 2014 Operating Partnership Series C Convertible Units	Jun. 30, 2013 Operating Partnership Series C Convertible Units	Jun. 30, 2014 Operating Partnership Series C Convertible Units Sponsor	Dec. 31, 2013 Operating Partnership Series C Convertible Units Sponsor	Jun. 30, 2014 Operating Partnership Series D Convertible Units Sponsor	Dec. 31, 2013 Operating Partnership Series D Convertible Units Sponsor	Jun. 30, 2014 Certain consolidated subsidiaries with noncontrolling interest	Jun. 30, 2014 Certain consolidated subsidiaries with noncontrolling interest
Class of Stock
Percentage of units outstanding				23.90%	24.60%				6.90%	6.90%
Units owned (in shares)									13,787,292	13,787,292					31,085,974	31,085,974	4,375,000	4,375,000
Units outstanding (in shares)						199,320,336	199,320,336	199,320,336
Income (loss) allocated to noncontrolling interest	$ 0.10	$ 0.20				$ (0.60)	$ (1.30)				$ 4.70	$ 4.70	$ 9.20	$ 5.50					$ 0.07	$ (0.02)
Rate of Preferred units	3.50%	3.50%	3.50%