Schedule III (Consolidated) - Real Estate and Accumulated Depreciation - Reconciliation of Real Estate Owned (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
SEC Schedule III, Reconciliation of Carrying Amount of Real Estate Investments [Roll Forward]
Balance at beginning of the year	$ 381,969	$ 377,668	$ 379,944
Amounts capitalized	39,261	13,875	13,372
Impairments	(714)	(357)	0
Cost of real estate sold	(14,274)	(6,489)	(14,584)
Amounts retired or adjusted	(1,866)	(2,728)	(1,064)
Balance at the end of the year	$ 404,376	$ 381,969	$ 377,668