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, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Cash and cash equivalents	$ 546,520,000	$ 488,683,000	$ 515,646,000	$ 443,420,000
Cash and Cash Equivalents, at Carrying Value	823,207,000	766,962,000	781,283,000	673,173,000
Restricted Cash and Cash Equivalents	276,687,000	278,279,000	265,637,000	229,753,000
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Cash and cash equivalents	$ 823,207,000	$ 766,962,000	$ 781,283,000	$ 673,173,000