Distributions (Details) - $ / shares	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Distributions
Minimum distribution of taxable income (as a percent)									90.00%
Ordinary taxable distribution									$ 1.607	$ 1.485	$ 1.690
Return of capital									0.444	0.556	0.357
Unrecaptured Section 1250 gain									0.163	0.149	0.023
Long-term capital gain									0.066
Total	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.57	$ 0.54	$ 0.54	$ 0.54	$ 2.28	$ 2.19	$ 2.07