Summary of Valuation of Company's Derivatives by Pricing Observability Levels (Parenthetical) (Detail) (Interest Rate Swaps, USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2013
Interest Rate Swaps
Fair Value Measurements, Recurring and Nonrecurring, Valuation Techniques [Line Items]
Interest rate derivative asset	$ 60

The increase (decrease) during the period in the carrying value of derivative instruments reported as liabilities that are due to be disposed of within one year (or the normal operating cycle, if longer).

No definition available.

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