Schedule-II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of year	$ 202
Additions Charged Against Operations	19,711
Uncollectible accounts Written-off	(48)
Balance at End of Year	19,865	$ 202
Allowance for Doubtful Accounts [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at Beginning of year	202	365	$ 333
Additions Charged Against Operations	123	315	87
Uncollectible accounts Written-off	(48)	(478)	(55)
Balance at End of Year	277	$ 202	$ 365
Allowance for Preferred Equity Investments [Member]
Valuation And Qualifying Accounts Disclosure [Line Items]
Additions Charged Against Operations	19,588
Balance at End of Year	$ 19,588