CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stockholders' Equity Attributable to Parent
Common dividends declared - per share (in dollars per share)	$ 2.72	$ 2.52	$ 2.44
Issuance of shares of common stock, common stock offering, net of expenses (in shares)	1,706,474	1,370,457	875,052
Issuance of shares of common stock, options exercised (in shares)	0	0	0
Issuance of shares of common stock, dividend reinvestment plan (in shares)	1,844	2,744	3,326
Withheld shares of common stock to satisfy tax withholding obligations in connection with the vesting of restricted stock (in shares)	23,824	33,695	57,316