Goodwill and other intangible assets - Total Amortizing Intangible Assets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	$ 933,062	$ 933,062
Accumulated Amortization	817,299	756,668
Net Carrying Amount	115,763	176,394
Core deposit [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	755,794	755,794
Accumulated Amortization	670,940	626,453
Net Carrying Amount	84,854	129,341
Other [Member]
Finite-Lived Intangible Assets [Line Items]
Gross Carrying Amount	177,268	177,268
Accumulated Amortization	146,359	130,215
Net Carrying Amount	$ 30,909	$ 47,053