Commitments and Contingencies - Commitments and Letters of Credit (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Commitment And Contingencies [Line Items]
Commitments and letters of credit	$ 1,232,212	$ 1,117,168
Commitments to Extend Credit [Member]
Commitment And Contingencies [Line Items]
Commitments and letters of credit	1,158,628	1,038,529
Standby Letters of Credit [Member]
Commitment And Contingencies [Line Items]
Commitments and letters of credit	$ 73,584	$ 78,639