Summary of Significant Accounting Policies -Changes in Accumulated Other Comprehensive Income/(Loss) by Component (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014
Equity [Abstract]
OCI before reclassifications, Tax	$ (2)	$ (1)
Amounts reclassified from accumulated OCI related to pension plan liabilities, Tax	$ (1)	$ (1)

Amount of tax (expense) benefit of reclassification adjustment from accumulated other comprehensive (income) loss related to pension and other postretirement defined benefit plans.

No definition available.

Name:	us-gaap_OtherComprehensiveIncomeLossReclassificationAdjustmentFromAOCIPensionAndOtherPostretirementBenefitPlansTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration