Derivative Financial Instruments - Credit-Risk Related Contingent Features (Detail) $ in Millions	Dec. 31, 2017 USD ($)
One-notch Downgrade [Member]
Derivative [Line Items]
Amount of additional collateral to be posted upon downgrade	$ 30
Two-notch Downgrade [Member]
Derivative [Line Items]
Amount of additional collateral to be posted upon downgrade	$ 31

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=84234895&loc=SL5624258-113959

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