Equity (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
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	131,499
Common stock shares issued through dividend reinvestment plan	2,396	2,142	2,434
Number of shares of stock grants withheld	53,453	45,090	55,523
Common shares issued under Employee Stock Purchase Plan	11,348
Number of shares forfeited	4,262
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%
Stock Issued During Period, Shares, New Issues	0
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	197,018	157,779	115,163
Non Employee Director [Member] | Deferred Compensation, Share-based Payments [Member]
Class of Stock [Line Items]
Common stock shares issued in connection with stock grants	44,327	46,589