Stockholders' equity (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Issuances of common stock
Dividends declared on common stock	$ 51,800		$ 153,669
Common Stock, Dividends, Per Share, Declared	$ 0.72	$ 0.68	$ 2.14	$ 1.93
Dividends declared on preferred stock			19,414
Preferred stock, shares authorized	100,000,000		100,000,000
Number of shares issued and outstanding	15,200,000		15,200,000
Number of "excess stock" authorized (in shares)			200,000,000
Number of excess stock authorized issued and outstanding (in shares)	0		0
Series D Preferred Stock
Issuances of common stock
Dividends declared on preferred stock	4,400
Dividends declared on preferred stock (dollar per share)	$ 0.4375
Series E Cumulative Redeemable Preferred Stock
Issuances of common stock
Dividends declared on preferred stock	$ 2,100
Dividends declared on preferred stock (dollar per share)	$ 0.403125