Stockholders' Equity - Dividends Declared (Details) - $ / shares	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Dividends declared (in dollars per share)	$ 0.46	$ 0.46	$ 0.46	$ 0.46	$ 0.46	$ 0.46	$ 0.46	$ 0.46
Common stock [Member]
Class of Stock [Line Items]
Dividends declared (in dollars per share)	$ 0.4211								$ 1.84	$ 1.84	$ 1.78
Preferred A Stock [Member]
Class of Stock [Line Items]
Dividends declared (in dollars per share)									1.1861	2.1564	2.1564
Preferred B Stock [Member]
Class of Stock [Line Items]
Dividends declared (in dollars per share)									2	2	$ 0.6333
Preferred C Stock [Member]
Class of Stock [Line Items]
Dividends declared (in dollars per share)									$ 2	$ 1