Distributions Paid and Payable (Details) (USD $) In Millions, except Share data, unless otherwise specified			1 Months Ended																		9 Months Ended			1 Months Ended	9 Months Ended				12 Months Ended			1 Months Ended	9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Common stock	Aug. 31, 2012 Common stock	Jul. 31, 2012 Common stock	Jun. 30, 2012 Common stock	May 31, 2012 Common stock	Apr. 30, 2012 Common stock	Mar. 31, 2012 Common stock	Feb. 29, 2012 Common stock	Jan. 31, 2012 Common stock	Sep. 30, 2011 Common stock	Aug. 31, 2011 Common stock	Jul. 31, 2011 Common stock	Jun. 30, 2011 Common stock	May 31, 2011 Common stock	Apr. 30, 2011 Common stock	Mar. 31, 2011 Common stock	Feb. 28, 2011 Common stock	Jan. 31, 2011 Common stock	Sep. 30, 2012 Common stock	Sep. 30, 2011 Common stock	Oct. 31, 2012 Common stock	Mar. 31, 2012 Class D Cumulative Redeemable Preferred Stock	Sep. 30, 2012 Class D Cumulative Redeemable Preferred Stock	Sep. 30, 2011 Class D Cumulative Redeemable Preferred Stock M	Sep. 30, 2012 Class E Cumulative Redeemable Preferred Stock M	Sep. 30, 2011 Class E Cumulative Redeemable Preferred Stock M	Dec. 31, 2011 Class E Cumulative Redeemable Preferred Stock	Oct. 31, 2012 Class E Cumulative Redeemable Preferred Stock	Dec. 31, 2006 Class E Cumulative Redeemable Preferred Stock	Mar. 31, 2012 Class F Cumulative Redeemable Preferred Stock	Sep. 30, 2012 Class F Cumulative Redeemable Preferred Stock M	Oct. 31, 2012 Class F Cumulative Redeemable Preferred Stock	Apr. 30, 2012 Class F Cumulative Redeemable Preferred Stock	Feb. 29, 2012 Class F Cumulative Redeemable Preferred Stock
Distributions paid and payable
Dividends paid per common share (in dollars per share)			$ 0.1511250	$ 0.1461250	$ 0.1461250	$ 0.1458125	$ 0.1458125	$ 0.1458125	$ 0.1455000	$ 0.1455000	$ 0.1455000	$ 0.1448750	$ 0.1448750	$ 0.1448750	$ 0.1445625	$ 0.1445625	$ 0.1445625	$ 0.1442500	$ 0.1442500	$ 0.1442500	$ 1.3173125	$ 1.3010625
Monthly distributions payable (in dollars per share)																							0.1514375							$ 0.140625				$ 0.138021
Preferred stock redeemed (in shares)																								5,100,000
Preferred stock, dividend rate (as a percent)																								7.375%	7.375%	7.375%	6.75%	6.75%	6.75%				6.625%
Dividends paid per preferred share (in dollars per share)																									$ 0.3841147	$ 1.3828131	$ 1.265625	$ 1.265625					$ 0.9983517
Preferred stock dividends paid																									$ 2.0	$ 7.1	$ 11.1	$ 11.1					$ 15.9
Period for which dividends are paid																										9	9	9					7
Preferred stock, shares issued	25,150,000	13,900,000																													8,800,000				1,400,000	14,950,000
Preferred stock, redemption price per share (in dollars per share)																								$ 25			$ 25					$ 25	$ 25
Preferred stock, purchased by underwriters (in shares)																																				1,950,000
Preferred stock, issuance price per share (in dollars per share)																																			$ 25.2863	$ 25
Period when initial dividend payments were made																																37 days