CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Stockholders' Equity Attributable to Parent
Common dividends declared - per share (in dollars per share)	$ 2.44	$ 2.34	$ 2.22
Issuance of shares of common stock, common stock offering, net of expenses (in shares)	875,052	106,751	1,246,400
Issuance of shares of common stock, options exercised (in shares)	0	0	0
Issuance of shares of common stock, dividend reinvestment plan (in shares)	3,326	4,536	3,626
Withheld shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock (in shares)	57,316	32,409	31,673