COVERED LOANS - Carrying Amount of Accretable Yield for Purchased Impaired and Nonimpaired Loans (Detail) (Covered Loans, USD $) In Thousands, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2012		Sep. 30, 2011		Sep. 30, 2012		Sep. 30, 2011
Covered Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Balance at beginning of period	$ 283,296	[1]	$ 421,781	[1]	$ 344,410	[1]	$ 509,945	[1]
Reclassification from non-accretable difference	2,338		17,311		25,780		50,517
Accretion	(21,730)		(31,168)		(71,674)		(97,447)
Other net activity	(11,749)	[2]	(4,878)	[2]	(46,361)	[2]	(59,969)	[2]
Balance at end of period	$ 252,155		$ 403,046		$ 252,155		$ 403,046

[1] Excludes loans with revolving privileges which are scoped out of FASB Topic 310-30 and certain loans which First Financial elected to treat under the cost recovery method

[2] Includes the impact of loan repayments and charge-offs