Equity (Details 2) - USD ($)	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Class of Stock [Line Items]
Common stock, shares authorized (in shares)	280,000,000	280,000,000		280,000,000
Preferred stock, shares authorized (in shares)	50,000,000	50,000,000		50,000,000
Common stock shares issued in connection with stock options exercised (in shares)		36,169
Common stock issued through the dividend reinvestment plan (in shares)		1,828	1,041
Number of shares of stock grants withheld (in shares)		59,019	53,011
Stock issued during period, shares, employee stock purchase plans		5,872
Stock issued during period, shares, share-based compensation, forfeited (in shares)		2,211
Issuance of common stock, net		$ 110,033,000	$ 12,804,000
Continuous Equity Program CEP IV
Class of Stock [Line Items]
Common stock shares issued	262,247	568,424
Maximum value of shares of common stock that can be sold (in dollars)	$ 1,000,000,000	$ 1,000,000,000
Shares issued, average price per share (in dollars per share)	$ 190.66	$ 188.39
Issuance of common stock, net	$ 49,250,000	$ 105,478,000
Common stock value, remaining to be authorized under continuous equity program	$ 892,915,000	$ 892,915,000
Maximum | Continuous Equity Program CEP IV
Class of Stock [Line Items]
Percentage of compensation received by sales agent		2.00%
Restricted Stock and Restricted Stock Converted From Performance Shares
Class of Stock [Line Items]
Equity instruments granted (in shares)		201,314	196,059