Available-for-Sale Securities - Unrealized Loss Position (Details) $ in Millions	Jun. 30, 2018 USD ($) security	Dec. 31, 2017 USD ($) security
Available-for-Sale Securities
Number of investments classified as available-for-sale securities in an unrealized loss position , twelve months or less	12	29
Number of investments classified as available-for-sale securities in an unrealized loss position, more than twelve months	0	0
Aggregate fair value of securities none of which had been in an unrealized loss position for more than twelve months \| $	$ 35.8	$ 95.7

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for less than 12 months.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant