Schedule of Location of Derivatives on Balance Sheet (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Interest rate swaps - Asset Derivatives found in Interest rate hedge asset	$ 13,626
Forward purchase commitments - Asset Derivatives found in Other Assets	6,904	5,452
Interest rate swaps - Asset Derivatives found in Interest rate hedge liability	147,954	243,945
Futures contracts - Asset Derivatives found in Futures contract liability	$ 44,593