Loans Receivable - Narrative (Details) $ in Thousands, £ in Millions	1 Months Ended	3 Months Ended			12 Months Ended
	Jun. 30, 2018 property	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($) property	Mar. 31, 2018 GBP (£) property	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Dec. 31, 2018 USD ($)	Dec. 31, 2017 GBP (£) property	Dec. 31, 2016 GBP (£) property
Loans Receivable:
Loans receivable, net | $		$ 86,139					$ 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	$ 131,000		£ 105	£ 105
Period of call-option retained					3 years	3 years
Number of properties in a purchase option | property					7	7		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