Schedule of Interest Rate Swap Agreements on Accumulated Other Comprehensive Income (Detail) (Interest Rate Swap, USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Interest Rate Swap
Derivative Instruments and Hedging Activities Disclosure [Line Items]
Beginning balance	$ (243,051,000)	$ (218,451,000)
Unrealized loss on interest rate swaps	(5,907,000)	(141,392,000)
Reclassification of net losses included in income statement	29,929,000	116,792,000
Ending balance	$ (219,029,000)	$ (243,051,000)