Distributions Paid and Payable (Detail) (USD $) In Millions, except Per Share data, unless otherwise specified	1 Months Ended																																				12 Months Ended
	Dec. 31, 2014	Nov. 30, 2014	Oct. 31, 2014	Sep. 30, 2014	Aug. 31, 2014	Jul. 31, 2014	Jun. 30, 2014	May 31, 2014	Apr. 30, 2014	Mar. 31, 2014	Feb. 28, 2014	Jan. 31, 2014	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, 2014	Dec. 31, 2013	Dec. 31, 2012
Common stock
Issuance of common stock
Dividends paid per common share (in dollars per share)	$ 0.1831042	$ 0.1831042	$ 0.1831042	$ 0.1827917	$ 0.1827917	$ 0.1827917	$ 0.1824792	$ 0.1824792	$ 0.1824792	$ 0.1821667	$ 0.1821667	$ 0.1821667	$ 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	$ 2.1916254	$ 2.1474587	$ 1.7716250
Federal income tax characterization of distributions paid or deemed to be paid per common share
Ordinary income (in dollars per share)																																					$ 1.6483522	$ 1.3153791	$ 1.3367481
Nontaxable distributions (in dollars per share)																																					$ 0.5432732	$ 0.8320796	$ 0.4348769
Monthly distributions payable (in dollars per share)	$ 0.1834167												$ 0.1821667												$ 0.1821667												$ 2.1916254	$ 2.1474587	$ 1.7716250
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
Preferred stock dividends declared																																							$ 2.0
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
Dividend paid per preferred share of stock (in dollars per share)																																					$ 1.4484375	$ 1.6875	$ 1.6875
Preferred stock dividends declared																																					12.7	14.9	14.9
Class F Cumulative Redeemable Preferred Stock
Issuance of common stock
Monthly distributions payable (in dollars per share)	$ 0.138021																																				$ 0.138021
Federal income tax characterization of distributions paid or deemed to be paid per common share
Monthly distributions payable (in dollars per share)																																					$ 0.138021
Dividend paid per preferred share of stock (in dollars per share)												$ 0.1380210																									$ 1.656252	$ 1.656252	$ 1.4124147
Preferred stock dividends declared																																					$ 27.1	$ 27.1	$ 22.6