Investments - Schedule of Activity in Real Estate Assets Held for Sale (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2018
USD ($)
Property
Number of Properties  
Beginning Balance | Property 15
Transfers from real estate investments held and used | Property 9
Sales | Property (6)
Transfers to real estate investments held in use | Property (7)
Transfers to SMTA | Property (5)
Impairments | Property 0
Ending Balance | Property 6
Carrying Value  
Beginning Balance | $ $ 48,929
Transfers from real estate investments held and used | $ 39,487
Sales | $ (10,257)
Transfers to real estate investments held in use | $ (25,715)
Transfers to SMTA | $ (7,853)
Impairments | $ (990)
Ending Balance | $ $ 43,601