Distributions Paid and Payable (Details) (USD $) In Millions, except Share data, unless otherwise specified			1 Months Ended												6 Months Ended				12 Months Ended		1 Months Ended	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Common stock	May 31, 2014 Common stock	Apr. 30, 2014 Common stock	Mar. 31, 2014 Common stock	Feb. 28, 2014 Common stock	Jan. 31, 2014 Common stock	Jun. 30, 2013 Common stock	May 31, 2013 Common stock	Apr. 30, 2013 Common stock	Mar. 31, 2013 Common stock	Feb. 28, 2013 Common stock	Jan. 31, 2013 Common stock	Jun. 30, 2014 Common stock	Jun. 30, 2013 Common stock	Jun. 30, 2014 Class E Cumulative Redeemable Preferred Stock M	Jun. 30, 2013 Class E Cumulative Redeemable Preferred Stock M	Dec. 31, 2006 Class E Cumulative Redeemable Preferred Stock	Dec. 07, 2011 Class E Cumulative Redeemable Preferred Stock	Feb. 29, 2012 Class F Cumulative Redeemable Preferred Stock	Jun. 30, 2014 Class F Cumulative Redeemable Preferred Stock M	Jun. 30, 2013 Class F Cumulative Redeemable Preferred Stock M	Apr. 30, 2012 Class F Cumulative Redeemable Preferred Stock
Distributions paid and payable
Dividends paid per common share (in dollars per share)			$ 0.1824792	$ 0.1824792	$ 0.1824792	$ 0.1821667	$ 0.1821667	$ 0.1821667	$ 0.1812292	$ 0.1812292	$ 0.1812292	$ 0.1809167	$ 0.1809167	$ 0.1517500	$ 1.0939377	$ 1.0572710
Monthly distributions payable (in dollars per share)			$ 0.1827917												$ 0.1827917		$ 0.140625					$ 0.138021
Preferred stock, shares issued	25,150,000	25,150,000																	8,800,000		14,950,000			1,400,000
Preferred stock, dividend rate (as a percent)																			6.75%		6.625%
Preferred stock, issuance price per share (in dollars per share)																			$ 25.00		$ 25.00			$ 25.2863
Preferred stock, redemption price per share (in dollars per share)																				$ 25.00		$ 25.00
Period for which dividends are paid																	6	6				6	6
Dividends paid per preferred share (in dollars per share)																	$ 0.84375	$ 0.84375				$ 0.828126	$ 0.828126
Preferred stock dividends paid																	$ 7.4	$ 7.4				$ 13.5	$ 13.5