Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Millions					3 Months Ended	6 Months Ended	12 Months Ended
	Oct. 15, 2017	Oct. 13, 2017 USD ($) $ / shares shares	Apr. 22, 2015 USD ($) $ / shares shares	Jun. 04, 2013 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Jun. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) director quarter $ / shares shares	Aug. 10, 2017 USD ($) $ / shares
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)					200,000,000	200,000,000	200,000,000
Preferred stock, par value (in dollars per share) | $ / shares					$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares issued (in shares)					12,000,000	12,000,000	12,000,000
Preferred stock, shares outstanding (in shares)							12,000,000
Common stock, par value (in dollars per share) | $ / shares					$ 0.01	$ 0.01	$ 0.01
Equity Distribution Agreements
Class of Stock [Line Items]
Shares issued (in shares)					12,145,144,000	12,145,144
Common stock, par value (in dollars per share) | $ / shares								$ 0.01
Common stock that may be sold (up to) | $								$ 100.0
Share price (in dollars per share) | $ / shares					$ 6.17	$ 6.17
Proceeds from issuance of common stock | $					$ 73.8		$ 73.8
Common stock reserved for issuance | $					$ 24.7	$ 24.7
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)					6,000,000	6,000,000	6,000,000
Preferred stock, par value (in dollars per share) | $ / shares				$ 0.01	$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares issued (in shares)					3,000,000	3,000,000	3,000,000
Preferred stock, shares outstanding (in shares)					3,000,000	3,000,000	3,000,000
Shares issued (in shares)				3,000,000
Preferred stock, cumulative redeemable dividend rate				7.75%		7.75%	7.75%
Preferred stock, liquidation preference per share (in dollars per share) | $ / shares				$ 25	$ 25.00	$ 25.00	$ 25
Preferred stock issuance, net | $				$ 72.4
Redemption term						120 days
Redemption price (in dollars per share) | $ / shares					$ 25.00	$ 25.00
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)					4,140,000	4,140,000	4,140,000
Preferred stock, par value (in dollars per share) | $ / shares			$ 0.01		$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares issued (in shares)			3,600,000		3,600,000	3,600,000	3,600,000
Preferred stock, shares outstanding (in shares)					3,600,000	3,600,000	3,600,000
Preferred stock, cumulative redeemable dividend rate			7.875%			7.875%	7.875%
Preferred stock, liquidation preference per share (in dollars per share) | $ / shares			$ 25		$ 25.00	$ 25.00	$ 25
Preferred stock issuance, net | $			$ 86.9
Redemption term						120 days
Redemption price (in dollars per share) | $ / shares					$ 25.00	$ 25.00
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares)					5,750,000	5,750,000	5,750,000
Preferred stock, par value (in dollars per share) | $ / shares		$ 0.01			$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares issued (in shares)		5,400,000			5,400,000	5,400,000	5,400,000
Preferred stock, shares outstanding (in shares)					5,400,000	5,400,000	5,400,000
Preferred stock, cumulative redeemable dividend rate	5.695%	8.00%				8.00%	8.00%
Preferred stock, liquidation preference per share (in dollars per share) | $ / shares		$ 25			$ 25	$ 25	$ 25
Preferred stock issuance, net | $		$ 130.5
Redemption term						120 days
Redemption price (in dollars per share) | $ / shares					$ 25.00	$ 25.00
Series B, Series C and Series D Preferred Stock
Class of Stock [Line Items]
Number of quarters with no dividends that results in voting rights | quarter							6
Number of elected directors pending preferred stock voting rights | director							2
Stockholders required for term changes, percentage					66.67%	66.67%