CONSOLIDATED STATEMENTS OF EQUITY (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Dividends declared and paid per common stock (in dollars per share)	$ 1.86	$ 1.78	$ 1.71
Stock issued during period, shares (in shares)	35,456	31,807	34,230
Discounted stock purchase program, shares issued (in shares)	13,695	8,444	12,065
ATM equity program, shares issued (in shares)	5,821,366	5,716,222	8,573,533
Restricted common stock, shares issued (in shares)	274,102	222,157	209,284
Series D Depositary Share
Preferred stock (in USD per share)	$ 0.312847	$ 1.65625	$ 1.65625
Stock issued during period, shares (in shares)	11,500,000
Series E Depositary Share
Preferred stock (in USD per share)	$ 1.42500	1.42500	$ 1.42500
Series F Preferred Stock
Preferred stock (in USD per share)	$ 1.30000	$ 0.231111
Stock issued during period, shares (in shares)		13,800,000