Stockholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Jul. 31, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Issuances of common stock
Proceeds from the issuance of common stock				$ 367,802	$ 5,052
Dividends declared on common stock		$ 62,400		$ 121,477
Dividends declared per share of common stock (usd per share)		$ 0.8	$ 0.77	$ 1.60	$ 1.51
Preferred stock, shares authorized		100,000,000		100,000,000
Number of shares issued and outstanding		12,800,000		12,800,000
Number of "excess stock" authorized (in shares)				200,000,000
Number of excess stock authorized issued and outstanding (in shares)		0		0
Stock Redeemed or Called During Period, Value				$ 59,310
Preferred Stock, Accretion of Redemption Discount		$ 9,473	$ 0	$ 12,519	$ 0
Series D Cumulative Convertible Preferred Stock
Issuances of common stock
Dividends declared on preferred stock		$ 3,400
Dividends declared on preferred stock (dollar per share)		$ 0.4375
Stock Redeemed or Called During Period, Shares		1,000,000		1,931,934
Treasury Stock, Value, Acquired, Cost Method		$ 33,700		$ 59,300
Treasury Stock Acquired, Average Cost Per Share		$ 33.69		$ 30.70
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
At the Market Common Stock Offering Program, Established December 2015
Issuances of common stock
Common Stock, Shares Authorized		450,000,000		450,000,000
At the Market Equity Offering Programs
Issuances of common stock
Issuances of common stock (shares)		3,655,256		3,948,491
Proceeds from Shares Sold Gross		$ 348,400		$ 374,300
Shares Issued, Average Price Per Share		$ 95.31		$ 94.80
Proceeds from the issuance of common stock		$ 342,500		$ 367,800
Additional Paid-In Capital | Series D Cumulative Convertible Preferred Stock
Issuances of common stock
Stock Redeemed or Called During Period, Value		$ (781)		$ (1,508)
Subsequent Event | Series D Cumulative Convertible Preferred Stock
Issuances of common stock
Stock Redeemed or Called During Period, Shares	1,100,000
Treasury Stock, Value, Acquired, Cost Method	$ 39,300
Treasury Stock Acquired, Average Cost Per Share	$ 36.31
Subsequent Event | Forward Contracts [Member]
Issuances of common stock
Sale of Stock, Price Per Share	$ 101.00
Proceeds from the issuance of common stock	$ 724,000
Total Shares [Member] | Subsequent Event | Forward Contracts [Member]
Issuances of common stock
Common Stock, Shares Authorized	7,500,000
Incremental Shares [Member] | Subsequent Event | Forward Contracts [Member]
Issuances of common stock
Common Stock, Shares Authorized	975,000