Commitments and Contingencies - Commitments and Letters of Credit (Detail) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,626,345	$ 1,717,530
Commitments to Extend Credit
Other Commitments [Line Items]
Commitments and letters of credit	1,546,164	1,619,854
Standby Letters of Credit
Other Commitments [Line Items]
Commitments and letters of credit	$ 80,181	$ 97,676