Distributions Paid and Payable (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended																																				12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2015	Nov. 30, 2015	Oct. 31, 2015	Sep. 30, 2015	Aug. 31, 2015	Jul. 31, 2015	Jun. 30, 2015	May. 31, 2015	Apr. 30, 2015	Mar. 31, 2015	Feb. 28, 2015	Jan. 31, 2015	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, 2015	Dec. 31, 2014	Dec. 31, 2013
Common stock
Issuance of common stock
Dividends paid per common share (in dollars per share)			$ 0.1905000	$ 0.1905000	$ 0.1905000	$ 0.1900000	$ 0.1900000	$ 0.1900000	$ 0.1895000	$ 0.1895000	$ 0.1895000	$ 0.1890000	$ 0.1890000	$ 0.1834167	$ 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	$ 2.2714167	$ 2.1916254	$ 2.1474587
Federal income tax characterization of distributions paid or deemed to be paid per common share
Ordinary income (in dollars per share)																																							1.7307023	1.6483522	1.3153791
Nontaxable distributions (in dollars per share)																																							0.5407144	0.5432732	0.8320796
Monthly distributions payable (in dollars per share)	$ 0.1910	$ 0.1834167																																					2.2714167	$ 2.1916254	$ 2.1474587
Class E Cumulative Redeemable Preferred Stock
Issuance of common stock
Preferred stock dividends paid																																								$ 12.7	$ 14.9
Federal income tax characterization of distributions paid or deemed to be paid per common share
Monthly distributions payable (in dollars per share)																	$ 0.140625
Liquidation preference amount equivalent to rate (in dollars per share)																																								$ 1.4484375	$ 1.6875
Class F Cumulative Redeemable Preferred Stock
Issuance of common stock
Monthly distributions payable (in dollars per share)	$ 0.138021		$ 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
Liquidation preference amount equivalent to rate (in dollars per share)																																							$ 1.656252	$ 1.656252	$ 1.656252
Preferred stock dividends declared																																							$ 27.1	$ 27.1	$ 27.1