Investment securities available- for-sale - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Proceeds from the sale of investment securities available-for-sale			$ 91,298,000	$ 0
Other than Temporary Impairment Losses, Investments	0	0	0	0
Available-for-sale Securities, Gross Unrealized Losses			93,190,000		130,434,000
Available-for-sale Securities, Amortized Cost Basis	5,745,437,000		5,745,437,000		5,345,877,000
AFS securities sold [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 94,200,000		$ 94,200,000