CONSOLIDATED STATEMENTS OF EQUITY (Parenthetical) - $ / shares	3 Months Ended							12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statement of Stockholders' Equity [Abstract]
Dividends to common stockholders, per share (in dollars per share)	$ 0.79	$ 0.79	$ 0.79	$ 0.79	$ 0.775	$ 0.775	$ 0.775	$ 3.1625	$ 3.115	$ 2.965