Distributions Paid and Payable (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended																																				12 Months Ended
	Dec. 31, 2013	Nov. 30, 2013	Oct. 31, 2013	Sep. 30, 2013	Aug. 31, 2013	Jul. 31, 2013	Jun. 30, 2013	May 31, 2013	Apr. 30, 2013	Mar. 31, 2013	Feb. 28, 2013	Jan. 31, 2013	Dec. 31, 2012	Nov. 30, 2012	Oct. 31, 2012	Sep. 30, 2012	Aug. 31, 2012	Jul. 31, 2012	Jun. 30, 2012	May 31, 2012	Apr. 30, 2012	Mar. 31, 2012	Feb. 29, 2012	Jan. 31, 2012	Dec. 31, 2011	Nov. 30, 2011	Oct. 31, 2011	Sep. 30, 2011	Aug. 31, 2011	Jul. 31, 2011	Jun. 30, 2011	May 31, 2011	Apr. 30, 2011	Mar. 31, 2011	Feb. 28, 2011	Jan. 31, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Common stock
Distributions paid and payable
Dividends paid per common share (in dollars per share)	$ 0.1818542	$ 0.1818542	$ 0.1818542	$ 0.1815417	$ 0.1815417	$ 0.1815417	$ 0.1812292	$ 0.1812292	$ 0.1812292	$ 0.1809167	$ 0.1809167	$ 0.1517500	$ 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	$ 2.1474587	$ 1.7716250	$ 1.7366250
Federal income tax characterization of distributions paid or deemed to be paid per common share
Ordinary income (in dollars per share)																																					$ 1.3153791	$ 1.3367481	$ 1.3787863
Nontaxable distributions (in dollars per share)																																					$ 0.8320796	$ 0.4348769	$ 0.3578387
Monthly distributions payable (in dollars per share)	$ 0.1821667												$ 0.15175																								$ 0.1821667	$ 0.15175
Class D Cumulative Redeemable Preferred Stock
Federal income tax characterization of distributions paid or deemed to be paid per common share
Dividend paid per preferred share of stock (in dollars per share)																						$ 0.1536459																$ 0.3841147	$ 1.8437508
Preferred stock dividends declared																																						$ 2.0	$ 9.4
Class E Cumulative Redeemable Preferred Stock
Federal income tax characterization of distributions paid or deemed to be paid per common share
Monthly distributions payable (in dollars per share)	$ 0.140625												$ 0.140625												$ 0.140625												$ 0.140625	$ 0.140625	$ 0.140625
Dividend paid per preferred share of stock (in dollars per share)																																					$ 1.6875	$ 1.6875	$ 1.6875
Preferred stock dividends declared																																					14.9	14.9	14.9
Class F Cumulative Redeemable Preferred Stock
Federal income tax characterization of distributions paid or deemed to be paid per common share
Monthly distributions payable (in dollars per share)	$ 0.138021												$ 0.138021																								$ 0.138021	$ 0.138021
Dividend paid per preferred share of stock (in dollars per share)																																					$ 1.656252	$ 1.4124147
Preferred stock dividends declared																																					$ 27.1	$ 22.6