General - Narrative (Detail) $ in Billions	Sep. 30, 2017 USD ($)
Summary of Investment Holdings [Line Items]
Short-term investments in U.S. Treasury Bills	$ 66.6
U.S. Treasury Bills [Member]
Summary of Investment Holdings [Line Items]
Investments	$ 80.6

Sum of the carrying amounts as of the balance sheet date of all investments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(h))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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