Stockholders' equity (Details) (USD $)	3 Months Ended	9 Months Ended			1 Months Ended	3 Months Ended	9 Months Ended	1 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended	3 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Mar. 31, 2012 Series E Preferred Stock	Sep. 30, 2012 Series E Preferred Stock	Sep. 30, 2012 Series E Preferred Stock	Apr. 30, 2012 8.375% Series C Preferred Stock	Dec. 31, 2011 8.375% Series C Preferred Stock	Jun. 30, 2012 Common Stock At-the-market equity offering programs	Sep. 30, 2012 Common Stock At-the-market equity offering programs	Sep. 30, 2012 Common Stock At-the-market equity offering programs	Sep. 30, 2012 Series D convertible preferred stock
Issuances of common stock
Aggregate value of the shares of common stock to be sold											$ 250,000,000	$ 250,000,000
Period of the shares of common stock to be sold										3 years
Number of shares issued through public offering					5,200,000
Number of shares issued through public offering											793,291	1,366,977
Aggregate gross proceeds from shares sold											59,500,000	100,000,000
Net proceeds from sale of common stock		98,443,000	451,539,000								58,500,000	98,400,000
Aggregate value of shares available for issuance											150,000,000	150,000,000
Percentage of dividend on preferred stock							6.45%
Payment of quarterly dividends in arrears at an annual rate (in dollars per share)							$ 1.6125
Redemption price per share (in dollars per share)						$ 25.00	$ 25.00	$ 25.00
Number of days within which preferred stock may be redeemed upon occurrence of change of control							120 days
Redemption of preferred stock (in shares)								5,185,500
Payment of accumulated and unpaid dividends (in dollars per share)								$ 0.5234375
Preferred stock redemption charge		5,978,000						6,000,000
Preferred stock , aggregate value						130,000,000	130,000,000	129,600,000	129,638,000
Price of shares issued (in dollars per share)					$ 25.00						$ 74.97	$ 73.15
Net proceeds from public offering (after deducting underwriters' discounts and other offering costs)					124,900,000
Cash dividend on common stock declared	33,800,000	96,103,000
Cash dividend on common stock declared (in dollars per share)	$ 0.53
Cash dividend on preferred stock declared		21,207,000				2,100,000							4,400,000
Cash dividend on preferred stock declared (in dollars per share)						$ 0.403125							$ 0.4375
Accumulated other comprehensive loss
Unrealized gain on marketable securities	3,090,000	3,090,000		3,834,000
Unrealized loss on interest rate hedge agreements	(25,336,000)	(25,336,000)		(32,980,000)
Unrealized gain (loss) on foreign currency translation	2,517,000	2,517,000		(5,365,000)
Total	$ (19,729,000)	$ (19,729,000)		$ (34,511,000)