Supplemental Disclosures of Cash Flow Information and Non-Cash Financing and Investing Activities Restricted Cash (Details) - USD ($)	6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and cash equivalents	$ 477,928,000	$ 600,296,000	$ 573,383,000	$ 515,646,000
Cash and Cash Equivalents, at Carrying Value	787,159,000	876,930,000	824,088,000	781,283,000
Restricted Cash and Cash Equivalents	309,231,000	276,634,000	250,705,000	265,637,000
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Cash and cash equivalents	$ 787,159,000	$ 876,930,000	$ 824,088,000	$ 781,283,000