Equity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Common stock, shares authorized	280,000,000	280,000,000
Preferred stock, shares authorized	50,000,000	50,000,000
Common stock shares issued in connection with stock options exercised	42,123
Common stock shares issued through dividend reinvestment plan	3,058	2,396	2,142
Number of shares of stock grants withheld	60,319	53,453	45,090
Common shares issued under Employee Stock Purchase Plan	11,528
Number of shares forfeited	3,388
Issuance of common stock, net	$ 111,093,000	$ 15,526,000	$ 690,184,000
Continuous Equity Program CEP IV [Member]
Class of Stock [Line Items]
Maximum value of shares of common stock that can be sold	$ 1,000,000,000
Percentage of compensation received by sales agent	2.00%
Issuance of common stock, net of withholdings (in shares)	568,424
Shares issued, weighted average price per share (in dollars per share)	$ 188.39
Issuance of common stock, net	$ 105,478,000
Common stock value, remaining to be authorized under continuous equity program	$ 892,915,000
Maximum [Member] | Continuous Equity Program CEP IV [Member]
Class of Stock [Line Items]
Percentage of compensation received by sales agent	2.00%
Restricted Stock Converted From Performance Shares [Member]
Class of Stock [Line Items]
Common stock shares issued in connection with stock grants	201,824	197,018	157,779