Commitments and Contingencies - Commitments and Letters of Credit (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,635,009	$ 1,232,212
Commitments to Extend Credit [Member]
Other Commitments [Line Items]
Commitments and letters of credit	1,538,397	1,158,628
Standby Letters of Credit [Member]
Other Commitments [Line Items]
Commitments and letters of credit	$ 96,612	$ 73,584