Goodwill and Other Intangible Assets - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Finite-Lived Intangible Assets [Line Items]
Goodwill	$ 428,234,000		$ 428,234,000
Goodwill, Other Changes	0	0
Net recoveries of impairment charges on its loan servicing rights	1,300,000	420,000
Impairment of core deposits and intangibles	0	0
Total amortization expense - other intangible assets	$ 1,600,000	$ 2,000,000
Core deposits [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	11 years
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted average amortization period, years	17 years