Stockholders' equity (Details) (USD $)	1 Months Ended		12 Months Ended			1 Months Ended		12 Months Ended		1 Months Ended	12 Months Ended		1 Months Ended
	May 31, 2013	May 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	May 31, 2013 Common Stock	May 31, 2011 Common Stock	Dec. 31, 2013 Common Stock At-the-market equity offering programs	Dec. 31, 2012 Common Stock At-the-market equity offering programs	Mar. 31, 2012 Series E Cumulative Redeemable Preferred Stock	Dec. 31, 2013 Series E Cumulative Redeemable Preferred Stock	Dec. 31, 2012 Series E Cumulative Redeemable Preferred Stock	Apr. 30, 2012 Series C Preferred Stock	Mar. 31, 2012 Series C Preferred Stock	Apr. 30, 2008 7.00% series D Convertible Preferred Stock	Dec. 31, 2012 7.00% series D Convertible Preferred Stock
Issuances of common stock
Stock issued in secondary offering (in shares)						7,590,000		1,400,000		5,200,000					10,000,000
Stock issued pursuant to the exercise of underwriters' option (in shares)						990,000
Shares issued price (dollars per share)						$ 73.50
Aggregate net proceeds from issuance of common stock	$ 534,469,000		$ 534,469,000	$ 97,890,000	$ 451,539,000		$ 451,500,000	$ 97,900,000
Number of shares issued through public offering		6,250,651
Number of shares issued upon full exercise of the underwriters over-allotment option							750,651
Price of shares issued (in dollars per share)							$ 75.50	$ 73.15		$ 25.00
Aggregate value of the shares of common stock to be sold									250,000,000
Aggregate gross proceeds from shares sold								100,000,000
Payments of stock issuance costs								2,100,000					129,600,000
Aggregate value of shares available for issuance								150,000,000
Net proceeds from public offering (after deducting underwriters' discounts and other offering costs)										124,900,000
Percentage of dividend on preferred stock										6.45%	6.45%	6.45%
Payment of quarterly dividends in arrears at an annual rate (in dollars per share)										$ 1.6125					$ 1.75
Redemption price per share											$ 25.00		$ 25.00
Number of days within which preferred stock may be redeemed upon occurrence of change of control											120 days
Number of share issued and outstanding													5,185,500
Payment of accumulated and unpaid dividends (in dollars per share)													$ 0.5234375
Preferred stock redemption charge			$ 0	$ 5,978,000	$ 0									$ 6,000,000
Percentage of the closing sale price per share of the entity's common stock that the then-applicable conversion price must exceed in order for the shares to be automatically converted															150.00%
Minimum number of trading days within 30 consecutive trading days during which the closing sale price per share of entity's common stock equals or exceeds the then-applicable conversion price for the shares to be automatically converted															20 days
Number of consecutive trading day period within which the closing sale price per share of entity's common stock equals or exceeds the then-applicable conversion price for at least 20 trading days for the shares to be automatically converted															30 days
Conversion rate at a option of holders (in shares)															0.2477
Preferred stock liquidation preference per share											$ 25	$ 25			$ 25.00	$ 25.00
Conversion rate which is equivalent to an initial conversion price per share of common stock (in dollars per share)															$ 100.93
Conversion rate dividend adjustment per quarter (in dollars per share)															$ 0.78
Conversion rate (in shares)																0.2480
Conversion price (in dollars per share)																$ 100.81
Preferred stock, shares authorized			100,000,000								5,200,000	5,200,000
Number of shares issued and outstanding			15,200,000								5,200,000	5,200,000
Number of "excess stock" authorized (in shares)			200,000,000
Number of excess stock authorized issued and outstanding (in shares)			0