Derivative Financial Instruments (Detail) (Derivatives Designated as Hedges, USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Asset derivatives fair value	$ 18,496	$ 26,081
Liability derivatives fair value	14,678	4,647
Accounts receivable-other
Derivative [Line Items]
Asset derivatives fair value	7,853	12,311
Other assets
Derivative [Line Items]
Asset derivatives fair value	10,643	13,770
Accounts payable
Derivative [Line Items]
Liability derivatives fair value	10,134	3,604
Other liabilities
Derivative [Line Items]
Liability derivatives fair value	$ 4,544	$ 1,043