Condensed Consolidated Balance Sheets (Parenthetical) - $ / shares	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Condensed Consolidated Balance Sheets [Abstract]
Preferred stock, dividend rate	6.50%	6.50%
Preferred stock, par value	$ 0.01	$ 0.01
Preferred stock, shares authorized	363,000	363,000
Preferred stock, shares issued	363,000	363,000
Preferred stock, shares outstanding	363,000	363,000
Common stock, par value	$ 0.01	$ 0.01
Common stock, shares authorized	500,000,000	500,000,000
Common stock, shares issued	104,947,759	103,099,692
Treasury stock, shares	14,622,930	11,751,726