Distributions Paid and Payable - Common Stock (Details) - Common stock - $ / shares	1 Months Ended																																				12 Months Ended
	Dec. 31, 2017	Nov. 30, 2017	Oct. 31, 2017	Sep. 30, 2017	Aug. 31, 2017	Jul. 31, 2017	Jun. 30, 2017	May 31, 2017	Apr. 30, 2017	Mar. 31, 2017	Feb. 28, 2017	Jan. 31, 2017	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	Jan. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Distributions Paid and Payable
Distributions characterized as ordinary income (in dollars per share)																																					$ 1.9402085	$ 1.8771975	$ 1.7307023
Distributions characterized as nontaxable distributions (in dollars per share)																																					0.5478464	0.5143025	0.5407144
Total capital gain distrubution (in dollars per share)																																					0.0389451
Total common stock dividends paid (in dollars per share)	$ 0.2120000	$ 0.2120000	$ 0.2120000	$ 0.2115000	$ 0.2115000	$ 0.2115000	$ 0.2110000	$ 0.2110000	$ 0.2110000	$ 0.2105000	$ 0.2105000	$ 0.2025000	$ 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	$ 0.1834167	2.5270000	2.3915000	$ 2.2714167
Monthly distributions payable (in dollars per share)	$ 0.2125												$ 0.2025																								$ 0.2125	$ 0.2025