Income taxes Reconciliation of net income to taxable income (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012		Dec. 31, 2011
Income Tax Disclosure [Abstract]
Net income	$ 140,249,000	$ 105,528,000		$ 135,393,000
Net income attributable to noncontrolling interests	(4,032,000)	(3,402,000)		(3,975,000)
Rental revenue recognition		11,607,000		(5,886,000)
Depreciation and amortization		15,501,000		(3,705,000)
Gains/losses from capital transactions		0		283,000
Stock-based compensation		11,488,000		8,249,000
Interest expense		(8,068,000)		(2,299,000)
Sales of property		394,000		(24,000)
Impairments		13,450,000		0
Other		2,268,000		4,124,000
Taxable income, before dividend deduction		148,766,000		132,160,000
Dividend deduction necessary to eliminate taxable income (1)		(148,766,000)	[1]	(132,160,000)	[1]
Estimated income subject to federal income tax		0		0
Distributions paid		$ 154,300,000		$ 135,200,000

[1] Total common stock and preferred stock dividend distributions paid were approximately $154.3 million and $135.2 million for the years ended December 31, 2012 and 2011, respectively.