Stockholders' equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jun. 30, 2017	Mar. 31, 2017
ATM program
Shares of common stock authorized	200,000,000
Proceeds from the issuance of common stock	$ 99,369		$ 217,759
Preferred stock
Preferred stock redemption charge	0		$ 11,279
Dividends declared on common stock	$ 91,980
Dividends declared per share of common stock (dollar per share)	$ 0.90		$ 0.83
Dividends declared on preferred stock	$ 1,302
Preferred stock, shares authorized	100,000,000
Shares of preferred stock issued and outstanding	2,975,432
Shares of common stock issued and outstanding	100,696,361
Number of "excess stock" authorized (in shares)	200,000,000
Number of excess stock authorized issued and outstanding (in shares)	0
7.00% Series D Cumulative Convertible Preferred Stock
Preferred stock
Preferred Stock, Shares Outstanding	2,975,432		2,975,432
Aggregate price on repurchase of Series D preferred stock			$ 17,934
Aggregate price per share on repurchase of Series D preferred stock			$ 35.79
Preferred stock redemption charge			$ 5,797
Dividends declared on preferred stock (dollar per share)	$ 0.4375
6.45% Series E Cumulative Redeemable Preferred Stock
Preferred stock
Number of shares repurchased/redeemed		5,200,000
Preferred stock redemption charge			$ 5,482
Preferred Stock, Redemption Date		Apr. 14, 2017
Preferred stock redemption liability		$ 130,000
Preferred Stock, Redemption Price Per Share		$ 25.00
ATM Common Stock Offering Program, Established August 2017
ATM program
Shares of common stock authorized	750,000,000
Issuances of common stock (shares)	2,773,318
Gross proceeds from issuance of common stock	$ 336,602
Shares Issued, Average Price Per Share	$ 121.37
Proceeds from the issuance of common stock	$ 331,154
Common stock value available for future issuance	413,398
Additional Paid-In Capital | 7.00% Series D Cumulative Convertible Preferred Stock
Preferred stock
Number of shares repurchased/redeemed			501,115
Repurchase/redemption of preferred stock			$ 391
Retained Earnings
Preferred stock
Dividends declared on common stock	91,980
Dividends declared on preferred stock	$ 1,302