Supplemental Disclosures of Cash Flow Information and Non-Cash Financing and Investing Activities Restricted Cash (Details) - USD ($)	9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and cash equivalents	$ 422,971,000	$ 546,520,000	$ 573,383,000	$ 515,646,000
Cash and Cash Equivalents, at Carrying Value	724,687,000	823,207,000	824,088,000	781,283,000
Restricted Cash and Cash Equivalents	301,716,000	276,687,000	250,705,000	265,637,000
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Cash and cash equivalents	$ 724,687,000	$ 823,207,000	$ 824,088,000	$ 781,283,000