Summary of Significant Accounting Policies - Cash Flows as Previously Reported and as Changed (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash flows provided by operating activities:
Restricted cash and cash equivalents	$ (151)	$ 9,137	$ 43,706
Net cash provided by operating activities	144,196	87,962	40,398
Cash flows used in investing activities:
Restricted funds held for derivative collateral and legal matters	(330)	(5,405)	5,960
Net cash used in investing activities	(131,546)	(75,140)	(68,100)
As Reported [Member]
Cash flows provided by operating activities:
Net cash provided by operating activities		97,099	84,104
Cash flows used in investing activities:
Restricted cash and cash equivalents		(3,732)	(49,666)
Net cash used in investing activities		(84,277)	(111,806)
As Changed [Member]
Cash flows provided by operating activities:
Restricted cash and cash equivalents		(9,137)	(43,706)
Net cash provided by operating activities		87,962	40,398
Cash flows used in investing activities:
Restricted funds held for derivative collateral and legal matters		5,405	(5,960)
Net cash used in investing activities		$ (75,140)	$ (68,100)

The net cash inflow or outflow for the increase (decrease) associated with funds that are not available for withdrawal or use (such as funds held in escrow) and are associated with underlying transactions that are classified as investing activities.

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The net cash from (used in) all of the entity's operating activities, including those of discontinued operations, of the reporting entity. Operating activities generally involve producing and delivering goods and providing services. Operating activity cash flows include transactions, adjustments, and changes in value that are not defined as investing or financing activities. While for technical reasons this element has no balance attribute, the default assumption is a debit balance consistent with its label.

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