FDIC loss share (expense) income - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
F D I C Loss Share Expense Income [Line Items]
F D I C Indemnification Asset Accretion Of Discount	$ (31,065,000)	$ (72,095,000)	$ (58,381,000)	$ (121,041,000)
Losses Incurred By The Corporation Not Claimed To Fdic [Member]
F D I C Loss Share Expense Income [Line Items]
F D I C Indemnification Asset Accretion Of Discount	$ 10,900,000
Federal Deposit Insurance Corporation [Member]
F D I C Loss Share Expense Income [Line Items]
Loss Sharing Agreement Allocation Percentage			80.00%