Goodwill and other intangible assets - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Goodwill And Other Intangible Assets [Line Items]
Useful lives of core deposit and other intangible assets	8 years
Remaining weighted-average amortization period	4 Years
Amortization of core deposit and other intangible assets	$ 60,631,000	$ 61,617,000	$ 58,103,000
Core deposit and other intangible assets [Member]
Goodwill And Other Intangible Assets [Line Items]
Amortization of core deposit and other intangible assets	$ 60,631,000	$ 61,617,000	$ 58,103,000
Minimum [Member]
Goodwill And Other Intangible Assets [Line Items]
Useful lives of core deposit and other intangible assets	5 years
Maximum [Member]
Goodwill And Other Intangible Assets [Line Items]
Useful lives of core deposit and other intangible assets	10 years