Supplemental Disclosures of Cash Flow Information and Non-Cash Financing and Investing Activities Restricted Cash (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Cash and cash equivalents	$ 558,248,000	$ 546,721,000	$ 573,383,000	$ 515,646,000
Cash and Cash Equivalents, at Carrying Value	826,377,000	826,089,000	824,088,000	781,283,000
Restricted Cash and Cash Equivalents	268,129,000	279,368,000	250,705,000	265,637,000
Cash and Cash Equivalents inclusive of Restricted Cash [Member]
Cash and cash equivalents	$ 826,377,000	$ 826,089,000	$ 824,088,000	$ 781,283,000