Stockholders' Equity - Narrative (Details) $ / shares in Units, $ in Millions						3 Months Ended	9 Months Ended	12 Months Ended
	Aug. 14, 2018 USD ($) shares	Oct. 15, 2017	Oct. 13, 2017 USD ($) $ / shares shares	Apr. 22, 2015 USD ($) $ / shares shares	Jun. 04, 2013 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) director quarter $ / shares shares	Sep. 10, 2018 USD ($)	Aug. 10, 2017 USD ($) $ / shares
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) | 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) | shares						12,000,000	12,000,000	12,000,000
Preferred stock, shares outstanding (in shares) | shares								12,000,000
Common stock, par value (in dollars per share) | $ / shares						$ 0.01	$ 0.01	$ 0.01
Underwritten Public Offering
Class of Stock [Line Items]
Shares issued (in shares) | shares	14,375,000						14,375,000
Share price (in dollars per share) | $ / shares						$ 6.16	$ 6.16
Proceeds from issuance of common stock | $	$ 86.0						$ 86.0
Equity Distribution Agreements
Class of Stock [Line Items]
Shares issued (in shares) | shares						2,443,487.00	14,588,631.00
Common stock, par value (in dollars per share) | $ / shares										$ 0.01
Common stock that may be sold (up to) | $									$ 177.1	$ 100.0
Proceeds from issuance of common stock | $						$ 15.2		$ 89.0
Common stock reserved for issuance | $						$ 86.4	$ 86.4
Series B Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares authorized (in shares) | 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) | shares						3,000,000	3,000,000	3,000,000
Preferred stock, shares outstanding (in shares) | shares						3,000,000	3,000,000	3,000,000
Shares issued (in shares) | 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	$ 25	$ 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) | 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) | shares				3,600,000		3,600,000	3,600,000	3,600,000
Preferred stock, shares outstanding (in shares) | 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) | 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) | shares			5,400,000			5,400,000	5,400,000	5,400,000
Preferred stock, shares outstanding (in shares) | 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			$ 25.00	$ 25.00	$ 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%
Minimum | Equity Distribution Agreements
Class of Stock [Line Items]
Share price (in dollars per share) | $ / shares						$ 6.31	$ 6.31
Maximum | Equity Distribution Agreements
Class of Stock [Line Items]
Share price (in dollars per share) | $ / shares						$ 6.19	$ 6.19