Distributions Paid and Payable (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended																																			3 Months Ended	12 Months Ended
	Dec. 31, 2016	Nov. 30, 2016	Oct. 31, 2016	Sep. 30, 2016	Aug. 31, 2016	Jul. 31, 2016	Jun. 30, 2016	May 31, 2016	Apr. 30, 2016	Mar. 31, 2016	Feb. 29, 2016	Jan. 31, 2016	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	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	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Common stock
Issuance of common stock
Dividends paid per common share (in dollars per share)	$ 0.2020000	$ 0.2020000	$ 0.2020000	$ 0.2015000	$ 0.1995000	$ 0.1995000	$ 0.1990000	$ 0.1990000	$ 0.1990000	$ 0.1985000	$ 0.1985000	$ 0.1910000	$ 0.1905000	$ 0.1905000	$ 0.1905000	$ 0.1900000	$ 0.1900000	$ 0.1900000	$ 0.1895000	$ 0.1895000	$ 0.1895000	$ 0.1890000	$ 0.1890000	$ 0.1831042	$ 0.1831042	$ 0.1831042	$ 0.1827917	$ 0.1827917	$ 0.1827917	$ 0.1824792	$ 0.1824792	$ 0.1824792	$ 0.1821667	$ 0.1821667	$ 0.1821667	$ 0.1834167	$ 2.3915000	$ 2.2714167	$ 2.1916254
Federal income tax characterization of distributions paid or deemed to be paid per common share
Ordinary income (in dollars per share)																																					1.8771975	1.7307023	1.6483522
Nontaxable distributions (in dollars per share)																																					0.5143025	0.5407144	0.5432732
Monthly distributions payable (in dollars per share)	0.2025												$ 0.1910000																								2.3915000	2.2714167	$ 2.1916254
Class E Cumulative Redeemable Preferred Stock
Issuance of common stock
Preferred stock dividends paid																																							$ 12.7
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
Class F 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
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