CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stockholders' Equity Attributable to Parent
Common dividends declared - per share (in dollars per share)	$ 2.34	$ 2.22	$ 2.14
Issuance of shares of common stock, common stock offering, net of expenses (in shares)	106,751	1,246,400	890,085
Issuance of shares of common stock, options exercised (in shares)	0	0	4,500
Issuance of shares of common stock, dividend reinvestment plan (in shares)	4,536	3,626	3,577
Withheld shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock (in shares)	32,409	31,673	9,412