Long-Term Debt - Additional Information (Detail) - USD ($) $ in Billions	Dec. 31, 2017	Dec. 31, 2016
Federal Home Loan Bank and Federal Reserve Bank [Member]
Debt Instrument [Line Items]
Unused Borrowing Capacity under Federal Home Loan Bank and Federal Reserve Bank	$ 87.7	$ 91.4

Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(b))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Name:	us-gaap_DebtInstrumentUnusedBorrowingCapacityAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant