Commitments and Contingencies - Commitments and Letters of Credit (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,467,727	$ 1,232,212
Commitments to Extend Credit [Member]
Other Commitments [Line Items]
Commitments and letters of credit	1,370,341	1,158,628
Standby Letters of Credit [Member]
Other Commitments [Line Items]
Commitments and letters of credit	$ 97,386	$ 73,584