CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01
Preferred stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	500,000,000	500,000,000
Common stock, shares issued (in shares)	228,254,255	225,321,660
Common stock, shares outstanding (in shares)	228,254,255	225,321,660
Series E Cumulative Redeemable Preferred Stock
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)	6.95%	6.95%
Preferred stock, Cumulative Redeemable Preferred Stock, shares issued (in shares)	4,600,000	4,600,000
Preferred stock, Cumulative Redeemable Preferred Stock, shares outstanding (in shares)	4,600,000	4,600,000
Preferred stock, Cumulative Redeemable Preferred Stock, liquidation preference (in dollars per share)	$ 25.00	$ 25.00
Series F Cumulative Redeemable Preferred Stock
Preferred stock, Cumulative Redeemable Preferred Stock, dividend rate (as a percent)	6.45%	6.45%
Preferred stock, Cumulative Redeemable Preferred Stock, shares issued (in shares)	3,000,000	3,000,000
Preferred stock, Cumulative Redeemable Preferred Stock, shares outstanding (in shares)	3,000,000	3,000,000
Preferred stock, Cumulative Redeemable Preferred Stock, liquidation preference (in dollars per share)	$ 25.00	$ 25.00