Stockholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Issuances of common stock
Proceeds from Shares Sold Gross	$ 5,300
Stock Issued Price Per Share	$ 94.02
Proceeds from the issuance of common stock			$ 5,052	$ 0
Payments of Stock Issuance Costs	$ 295
Dividends declared on common stock	$ 55,800		$ 164,806
Dividends declared per share of common stock (usd per share)	$ 0.77	$ 0.72	$ 2.28	$ 2.14
Preferred stock, shares authorized	100,000,000		100,000,000
Number of shares issued and outstanding	14,700,000		14,700,000
Number of "excess stock" authorized (in shares)			200,000,000
Number of excess stock authorized issued and outstanding (in shares)	0		0
Series D Cumulative Convertible Preferred Stock
Issuances of common stock
Dividends declared on preferred stock	$ 4,200
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
Common Stock
Issuances of common stock
Issuances of common stock (shares)			56,874