Consolidated Statements of Changes in Equity (Parenthetical) - $ / shares	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Common stock, par value (usd per share)	$ 0.01	$ 0.01
Treasury stock, par value (usd per share)	0.01	0.01
Common Stock
Common stock, par value (usd per share)	0.01	0.01	$ 0.01
Contingent Convertible Preferred Stock
Contingent convertible preferred stock, par value (usd per share)	0.01	0.01	0.01
Treasury Stock
Treasury stock, par value (usd per share)	$ 0.01	$ 0.01	$ 0.01