Derivatives Interest Rate Swap Agreements - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011
Derivative [Line Items]
Effective hedge losses reclassified into earnings		$ 141,392	$ 273,389
Ineffective losses recorded on a current basis in earnings		(178)	(654)
Estimated amount related to derivatives reclassified to interest expense during the next 12 months		118,769
Future cash flows for forecasted transactions, average period in months		31 months
Maximum leverage ratio the Company can reach before it could be in default of one of its derivative obligations	10 to 1
Fair value of derivatives in net liability position, which includes accrued interest but excludes any adjustment for nonperformance risk, related to agreements		$ 243,945