Derivative Financial Instruments - Additional Information (Detail) (USD $)	Jun. 30, 2013	Dec. 31, 2012
Derivative [Line Items]
Fair value derivative, assets	$ 229,383,000	$ 949,000
Fair value derivative, liabilities	58,606,000	1,102,000
Collateral paid for derivatives	124,759,000
Forward Sales Commitments [Member]
Derivative [Line Items]
Fair value derivative, assets	163,199,000
Fair value derivative, liabilities	10,113,000	1,102,000
Derivatives Subject to Master Netting Arrangement [Member] | Forward Sales Commitments [Member]
Derivative [Line Items]
Fair value derivative, assets	6,500,000
Fair value derivative, liabilities	3,600,000
Collateral paid for derivatives	2,700,000
Fair value derivative, net	$ 200,000