Distributions Paid and Payable (Details) $ / shares in Units, $ in Millions	1 Months Ended												6 Months Ended
	Jun. 30, 2015 $ / shares	May. 31, 2015 $ / shares	Apr. 30, 2015 $ / shares	Mar. 31, 2015 $ / shares	Feb. 28, 2015 $ / shares	Jan. 31, 2015 $ / shares	Jun. 30, 2014 $ / shares	May. 31, 2014 $ / shares	Apr. 30, 2014 $ / shares	Mar. 31, 2014 $ / shares	Feb. 28, 2014 $ / shares	Jan. 31, 2014 $ / shares	Jun. 30, 2015 USD ($) item $ / shares	Jun. 30, 2014 USD ($) item $ / shares	Oct. 31, 2014 $ / shares
Common stock
Issuance of common stock
Dividends paid per common share (in dollars per share)	$ 0.1895000	$ 0.1895000	$ 0.1895000	$ 0.1890000	$ 0.1890000	$ 0.1834167	$ 0.1824792	$ 0.1824792	$ 0.1824792	$ 0.1821667	$ 0.1821667	$ 0.1821667	$ 1.1299167	$ 1.0939377
Monthly distributions payable (in dollars per share)	0.19												0.19
Class E Cumulative Redeemable Preferred Stock
Issuance of common stock
Monthly distributions payable (in dollars per share)															$ 0.140625
Period for which dividends are paid | item														6
Dividends paid per preferred share (in dollars per share)														$ 0.843750
Preferred stock dividends paid | $														$ 7.4
Class F Cumulative Redeemable Preferred Stock
Issuance of common stock
Monthly distributions payable (in dollars per share)	$ 0.138021												$ 0.138021
Period for which dividends are paid | item													6	6
Dividends paid per preferred share (in dollars per share)													$ 0.828126	$ 0.828126
Preferred stock dividends paid | $													$ 13.5	$ 13.5