Fair Value Measurements Narrative (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Cash and cash equivalents	$ 316,264	$ 104,839
Money Market Funds [Member]
Cash and cash equivalents	$ 309,100	$ 96,700

Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes 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.

No definition available.

Name:	us-gaap_CashAndCashEquivalentsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant