Investments - Schedule of Activity in Real Estate Assets Held for Sale (Details)
$ in Thousands
9 Months Ended
Sep. 30, 2017
USD ($)
Property
Number of Properties  
Balance, December 31, 2016 (in properties) | Property 44
Transfers from real estate investments held and used (in properties) | Property 80
Sales (in properties) | Property (78)
Transfers to real estate investments held and used (in properties) | Property (8)
Balance, September 30, 2017 (in properties) | Property 38
Carrying Value  
Balance, December 31, 2016 $ 160,570
Transfers from real estate investments held and used 212,726
Sales (157,156)
Transfers to real estate investments held and used (63,597)
Impairments (19,161)
Balance, September 30, 2017 $ 133,382