Investment Activity - 2018 Investment Activity - Additional Information (Details) $ in Millions	12 Months Ended
Dec. 31, 2018 USD ($) Property Investment
Investment Activity [Abstract]
Number of oriented Healthcare-related real estate debt investments \| Investment	4
Number of real estate properties acquired \| Property	2
Additional investment on origination of funded additional in existing mortgage notes receivable \| $	$ 64.5

Additional investment on origination of funded additional mortgage loans on real estate.

No definition available.

Name:	mrt_AdditionalInvestmentOnOriginationOfFundedAdditionalMortgageLoansOnRealEstate
Namespace Prefix:	mrt_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant