Loans Receivable - Narrative (Details) $ in Thousands, £ in Millions	1 Months Ended			3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2018 property	Mar. 31, 2018 USD ($) property	Mar. 31, 2018 GBP (£) property	Mar. 31, 2018 USD ($)	Mar. 31, 2018 GBP (£)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2016 USD ($) property	Dec. 31, 2018 USD ($)	Dec. 31, 2016 GBP (£) property
Loans Receivable:
Loans receivable, net | $						$ 137,619			$ 62,998
Notes reduction due to call option exercised | $						$ 0	$ 147,474
MMCG
Loans Receivable:
Loss from initial consolidation of VIE				$ 41,000	£ 29
Bridge Loan | MMCG
Loans Receivable:
Loans receivable, net								$ 131,000		£ 105
Period of call-option retained								3 years
Number of properties in a purchase option (in properties) | property								7		7
Number of properties acquired in a purchase option | property	7	7	7
Notes reduction due to call option exercised | £			£ 105
Loss from initial consolidation of VIE		$ 41,000	£ 29