Investments - Schedule of Activity in Real Estate Assets Held for Sale (Details)
$ in Thousands
6 Months Ended
Jun. 30, 2018
USD ($)
Property
Number of Properties  
Beginning Balance (in properties) | Property 15
Transfers from real estate investments held and used (in properties) | Property 6
Sales (in properties) | Property (6)
Transfers to real estate investments held and used (in properties) | Property (7)
Transfer to SMTA | Property (5)
Ending Balance (in properties) | Property 3
Carrying Value  
Beginning Balance $ 48,929
Transfers from real estate investments held and used 14,090
Sales (10,257)
Transfers to real estate investments held in use (25,715)
Transfer to SMTA (7,853)
Impairments (369)
Ending Balance $ 18,825