Stockholders' Equity - Cash Dividends Declared - Preferred Stock (Details) - $ / shares	3 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Series B Preferred Stock
Dividends Payable [Line Items]
Cash Dividend Per Share (in dollars per share)	$ 0.484375	$ 0.484375	$ 0.484375	$ 0.484375	$ 0.484375	$ 0.484375	$ 0.484375
Series C Preferred Stock
Dividends Payable [Line Items]
Cash Dividend Per Share (in dollars per share)	0.4921875	0.4921875	0.4921875	0.4921875	$ 0.4921875	$ 0.4921875	$ 0.4921875
Series D Preferred Stock
Dividends Payable [Line Items]
Cash Dividend Per Share (in dollars per share)	$ 0.5	$ 0.5	$ 0.50	$ 0.51111