Notional Amounts and Unrealized Gains (losses) of Interest Rate Swap on Gross Basis, Amount Offset and Net Amounts (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended			3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Mar. 31, 2013 Interest Rate Swaps - Liability	Dec. 31, 2012 Interest Rate Swaps - Liability	Mar. 31, 2013 Interest Rate Swaps - Asset	Dec. 31, 2012 Interest Rate Swaps - Asset
Derivative Instruments Notional And Fair Value [Line Items]
Gross amounts - notional				$ 46,312,800	$ 45,811,800	$ 1,910,000	$ 1,100,000
Gross amounts -unrealized losses -liability				2,305,785	2,610,927
Amounts offset - notional				1,910,000	1,100,000	(1,910,000)	(1,100,000)
Amounts offset -unrealized losses -libility				(46,612)	(26,020)
Netted amounts -notional	48,222,800		46,911,800	48,222,800	46,911,800
Gross amounts -unrealized gains -assets						46,612	26,020
Amounts offset -unrealized gains -assets						(46,612)	(26,020)
Netted amounts -unrealized gain (losses)	$ 325,734	$ 341,639		$ 2,259,173	$ 2,584,907