Distributions - Federal Income Tax Classification of Per Share Common Stock Distributions (Details) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Distributions [Abstract]
Ordinary taxable distributions	$ 0.29	$ 0.66	$ 0.22
Return of capital	0.34	0.18	0.70
Total	$ 0.63	$ 0.84	$ 0.92

Return of capital distributions to common stockholders per share.

No definition available.

Name:	mrt_ReturnOfCapitalDistributionsToCommonStockholdersPerShare
Namespace Prefix:	mrt_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant