CONSOLIDATED BALANCE SHEETS (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Class of Stock [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized	1,000,000,000	1,000,000,000
Common stock, shares issued	451,617,000	437,729,000
Common stock, shares outstanding	441,060,000	428,820,000
Treasury stock, shares	10,557,000	8,909,000
Series B Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Class of Stock [Line Items]
Preferred stock, dividend rate, percentage	5.50%	5.50%
Preferred stock, shares issued	1,375,000	1,375,000
Preferred stock, shares outstanding	0	1,375,000
Preferred stock, aggregate liquidation value	$ 0.0	$ 1.4
Preferred stock
Class of Stock [Line Items]
Preferred stock, par value (in dollars per share)	$ 0.01
Preferred stock, shares authorized	20,000,000	20,000,000
Preferred stock | Series A Preferred Stock | Convertible Preferred Stock Subject to Mandatory Redemption
Class of Stock [Line Items]
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, dividend rate, percentage	5.25%