Subsequent events (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		1 Months Ended	0 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Apr. 30, 2015 property	May 07, 2015	Apr. 30, 2015	Dec. 31, 2014
Subsequent Event
Aggregate purchase price of properties acquired	$ 292,735	$ 341,064
Credit facility, additional borrowings, amount	346,000	321,000
Credit facility, repayment, amount	508,000	25,000
Total debt	45,000					207,000
Subsequent Events | Subsequent acquisitions
Subsequent Event
Number of properties acquired			213		213
Aggregate purchase price of properties acquired			28,400
Subsequent Events | 5.50% Series C participating preferred shares
Subsequent Event
Quarterly dividend (in dollars per share)				$ 0.34375
Rate of Preferred units				5.50%
Subsequent Events | Class A common shares
Subsequent Event
Quarterly dividend (in dollars per share)					$ 0.05
Subsequent Events | Class B common shares
Subsequent Event
Quarterly dividend (in dollars per share)				$ 0.05
Subsequent Events | Series A Preferred Stock
Subsequent Event
Quarterly dividend (in dollars per share)				$ 0.3125
Subsequent Events | Series B Preferred Stock
Subsequent Event
Quarterly dividend (in dollars per share)				$ 0.3125
Rate of Preferred units				5.00%
Subsequent Events | Senior Secured Revolving Credit Facility
Subsequent Event
Credit facility, additional borrowings, amount			89,000
Credit facility, repayment, amount			40,000
Total debt			$ 94,000		$ 94,000