Income taxes - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012 State	Dec. 31, 2011	Dec. 31, 2010
Schedule Of Income Taxes [Line Items]
Tax bad debt reserve	$ 79,121,000
Income taxes attributable to gains or losses on bank investment securities	18,766,000	28,712,000	32,778,000
Minimum alternative tax expense recognized for gains or losses on bank investment securities	$ 0	$ 0	$ 0
Number of state and local jurisdictions where company files income tax returns	40