Net Investment in Direct Financing Leases - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) property	Dec. 31, 2016 USD ($) property	Dec. 31, 2015 USD ($)	Sep. 30, 2013 property
Capital Leased Assets [Line Items]
Properties subject to direct financing leases | property	29	30
Income from direct financing leases	$ 54,217	$ 59,580	$ 61,000
Net investment in direct financing leases	714,352	752,589
DFL Portfolio
Capital Leased Assets [Line Items]
Income from direct financing leases	13,000	13,000	15,000
Cash income	18,000	18,000	$ 20,000
DFL Portfolio | Senior housing
Capital Leased Assets [Line Items]
Properties subject to direct financing leases | property				14
Net investment in direct financing leases	$ 356,000	$ 361,000