Distributions Paid and Payable - Common Stock (Details) - $ / shares	1 Months Ended																																			12 Months Ended			85 Months Ended
	Dec. 31, 2018	Nov. 30, 2018	Oct. 31, 2018	Sep. 30, 2018	Aug. 31, 2018	Jul. 31, 2018	Jun. 30, 2018	May 31, 2018	Apr. 30, 2018	Feb. 28, 2018	Jan. 31, 2018	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, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2018
Distributions Paid and Payable
Monthly distributions payable (in dollars per share)	$ 0.2210											$ 0.2125																								$ 0.2210	$ 0.2125
Common stock
Distributions Paid and Payable
Distributions characterized as ordinary income (in dollars per share)																																				2.0269173	1.9402085	$ 1.8771975
Distributions characterized as nontaxable distributions (in dollars per share)																																				0.6035827	0.5478464	0.5143025
Total capital gain distrubution (in dollars per share)																																				0.0000000	0.0389451	0.0000000
Total common stock dividends paid (in dollars per share)	$ 0.2205	$ 0.2205	$ 0.2205	$ 0.2200	$ 0.2200	$ 0.2200	$ 0.2195	$ 0.2195	$ 0.2195	$ 0.2190	$ 0.2125	$ 0.2120	$ 0.2120	$ 0.2120	$ 0.2115	$ 0.2115	$ 0.2115	$ 0.2110	$ 0.2110	$ 0.2110	$ 0.2105	$ 0.2105	$ 0.2025	$ 0.2020	$ 0.2020	$ 0.2020	$ 0.2015	$ 0.1995	$ 0.1995	$ 0.1990	$ 0.1990	$ 0.1990	$ 0.1985	$ 0.1985	$ 0.1910	$ 2.6305000	$ 2.5270000	$ 2.3915000	$ 0.2190