CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY (Parentheticals) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Increase (Decrease) In Stockholders' Equity [Roll Forward]
Balance (in shares)	198,309	196,142	187,399
Balance (in units)	8,772	8,862	8,956
Grant of restricted stock (in shares)	35	32	18
Grant of restricted stock (in dollars per share)	$ 30.51	$ 31.23	$ 33.09
Vesting/exercising of equity compensation plan (in shares)	89	116	773
Dividend reinvestment plan (in shares)	1,549	1,199	7,215
Dividend reinvestment plan (in dollars per share)	$ 30.22	$ 30.64	$ 33.27
Grant of stock as payment of directors fees (in shares)		5	10
Grant of stock as payment of fees (in dollars per share)		$ 32.18	$ 31.27
Deferred compensation directors (in shares)	35	8
Deferred compensation directors (in dollars per share)	$ 30.42	$ 32.10
Equity shelf program (in shares)	2,276	718	656
Equity shelf program (in dollars per share)	$ 33.18	$ 30.81	$ 29.97
Common dividends (in dollars per share)	$ 2.64	$ 2.54	$ 2.36
Conversion of OP Units to Common stock (in shares)	53	89	72
Conversion of OP Units to Common stock (in dollars per share)	$ 32.98	$ 32.91	$ 35.68
Redemption of OP Units (in units)	58	90	94
Balance (in shares)	202,346	198,309	196,142
Balance (in units)	8,714	8,772	8,862