Derivatives and Fair Value Measurements - Paid to and Received from Counterparties for Settlements of Matured Swaps (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Paid to counterparties	$ 342	$ 1,214	$ 2,260
Received from counterparties	$ 7	$ 915	$ 1,549

The cash inflow provided by derivative instruments during the period, which are classified as investing activities, excluding those designated as hedging instruments.

No definition available.

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