Other short-term borrowings (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Short-term Debt [Line Items]
Advances with the FHLB paying interest at maturity, at fixed rates ranging from 0.37% to 0.40%	$ 750,000	$ 295,000	$ 250,000
Term funds purchased paying interest at maturity, at fixed rates ranging from 0.70% to 1.05%	0	0	39,102
Others	1,200	1,200	1,200
Total other short-term borrowings	$ 751,200	$ 296,200	$ 290,302