Cash and Cash Equivalents, Restricted Cash and Marketable Securities (Details Textual) - USD ($)	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash and Cash Equivalents [Abstract]
Restricted cash	$ 102,061,000		$ 113,139,000
Line Of Credit Facility, Maximum Borrowing Capacity	1,800,000,000
Realized gains or losses on the sale of marketable securities	0	$ 0
Cash and Cash Equivalents, at Carrying Value	$ 330,061,000	$ 374,006,000	$ 371,167,000	$ 185,665,000