Distributions Paid and Payable (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended																																				12 Months Ended			1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2012 Common stock	Nov. 30, 2012 Common stock	Oct. 31, 2012 Common stock	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	Dec. 31, 2011 Common stock	Nov. 30, 2011 Common stock	Oct. 31, 2011 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	Dec. 31, 2010 Common stock	Nov. 30, 2010 Common stock	Oct. 31, 2010 Common stock	Sep. 30, 2010 Common stock	Aug. 31, 2010 Common stock	Jul. 31, 2010 Common stock	Jun. 30, 2010 Common stock	May 31, 2010 Common stock	Apr. 30, 2010 Common stock	Mar. 31, 2010 Common stock	Feb. 28, 2010 Common stock	Jan. 31, 2010 Common stock	Dec. 31, 2012 Common stock	Dec. 31, 2011 Common stock	Dec. 31, 2010 Common stock	Mar. 31, 2012 Class D Cumulative Redeemable Preferred Stock	Dec. 31, 2012 Class D Cumulative Redeemable Preferred Stock	Dec. 31, 2011 Class D Cumulative Redeemable Preferred Stock	Dec. 31, 2010 Class D Cumulative Redeemable Preferred Stock	Jan. 31, 2013 Class E Cumulative Redeemable Preferred Stock	Dec. 31, 2012 Class E Cumulative Redeemable Preferred Stock	Dec. 31, 2011 Class E Cumulative Redeemable Preferred Stock	Dec. 31, 2010 Class E Cumulative Redeemable Preferred Stock	Jan. 31, 2013 Class F Cumulative Redeemable Preferred Stock	Dec. 31, 2012 Class F Cumulative Redeemable Preferred Stock
Distributions paid and payable
Dividends paid per common share (in dollars per share)	$ 0.1514375	$ 0.1514375	$ 0.1514375	$ 0.1511250	$ 0.1461250	$ 0.1461250	$ 0.1458125	$ 0.1458125	$ 0.1458125	$ 0.1455000	$ 0.1455000	$ 0.1455000	$ 0.1451875	$ 0.1451875	$ 0.1451875	$ 0.1448750	$ 0.1448750	$ 0.1448750	$ 0.1445625	$ 0.1445625	$ 0.1445625	$ 0.1442500	$ 0.1442500	$ 0.1442500	$ 0.1439375	$ 0.1439375	$ 0.1439375	$ 0.1436250	$ 0.1436250	$ 0.1436250	$ 0.1433125	$ 0.1433125	$ 0.1433125	$ 0.143000	$ 0.143000	$ 0.143000	$ 1.771625	$ 1.736625	$ 1.721625
Federal income tax characterization of distributions paid or deemed to be paid per common share
Ordinary income (in dollars per share)																																					$ 1.3367481	$ 1.3787863	$ 1.2598879
Nontaxable distributions (in dollars per share)																																					$ 0.4348769	$ 0.3578387	$ 0.4617371
Monthly distributions payable (in dollars per share)	$ 0.15175												$ 0.1455																								$ 0.15175	$ 0.1455							$ 0.140625				$ 0.138021
Monthly distributions paid (in dollars per share)																																								$ 0.3841147		$ 1.8437508		$ 0.140625	$ 1.6875	$ 1.6875	$ 1.6875	$ 0.138021	$ 1.4124147
Preferred stock dividends declared																																								$ 2.0	$ 2.0	$ 9.4	$ 9.4		$ 14.9	$ 14.9	$ 14.9		$ 22.6
Federal income tax characterization of distributions paid per preferred share
Dividends paid per share (in dollars per share)																																								$ 0.15175	$ 0.3841147	$ 1.8437508	$ 1.8437508			$ 1.6875	$ 1.6875