Investments - Summary of Activity in Real Estate Assets Held for Sale (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
Property
Dec. 31, 2016
USD ($)
Property
Number of Properties    
Balance, beginning of period (in properties) | Property 44 36
Transfers from real estate investments (in properties) | Property 82 72
Sales (in properties) | Property (91) (59)
Transfers to real estate investments held and used (in properties) | Property (20) (5)
Balance, end of period (in properties) | Property 15 44
Carrying Value    
Balance, beginning of period $ 160,570 $ 84,259
Transfers from real estate investments 216,502 246,730
Sales (208,029) (126,100)
Transfers to real estate investments held and used (95,382) (21,046)
Impairments (24,732) (23,273)
Balance, end of period $ 48,929 $ 160,570