Stockholders' Equity - Dividends Declared (Details) - $ / shares	3 Months Ended				9 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Class of Stock [Line Items]
Dividends declared (in dollars per share)	$ 0.46			$ 0.46	$ 1.38	$ 1.38
Common stock [Member]
Class of Stock [Line Items]
Dividends declared (in dollars per share)	0.46	$ 0.46	$ 0.46
Series B Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared (in dollars per share)	0.5	0.5	0.5
Series C Preferred Stock [Member]
Class of Stock [Line Items]
Dividends declared (in dollars per share)	$ 0.5	$ 0.5	$ 0.5