Cash Equivalents and Investments - Related Party Transactions (Details) - USD ($) shares in Millions, $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Related Party Transaction [Line Items]
Cash, cash equivalents, and short-term investments	$ 208.4	$ 282.9
Common stock outstanding, held by JP Morgan Chase (shares)	3.7
Percent of outstanding common stock held by JP Morgan Chase during the period	10.03%
J.P. Morgan Securities LLC [Member]
Related Party Transaction [Line Items]
Cash, cash equivalents, and short-term investments, fair value	$ 97.2	$ 150.7
Percent of outstanding common stock held by JP Morgan Chase during the period	10.03%

Cash includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the customer may deposit additional funds at any time and effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid Investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. Short-term investments, exclusive of cash equivalents, generally consist of marketable securities intended to be sold within one year (or the normal operating cycle if longer) and may include trading securities, available-for-sale securities, or held-to-maturity securities (if maturing within one year), as applicable.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.1)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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