Summary of Significant Accounting Policies (Cash and Cash Equivalents) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Accounting Policies [Abstract]
Cash and cash equivalents	$ 526,241	$ 814,354
Overnight money market funds	461,700	752,400
Restricted cash	$ 55,507	$ 7,165

Amount of cash and cash equivalents restricted as to withdrawal or usage. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
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Name:	us-gaap_RestrictedCashAndCashEquivalents
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant