Commitments and Contingencies - Commitments and Letters of Credit (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,665,372	$ 1,717,530
Commitments to Extend Credit
Other Commitments [Line Items]
Commitments and letters of credit	1,574,971	1,619,854
Standby Letters of Credit
Other Commitments [Line Items]
Commitments and letters of credit	$ 90,401	$ 97,676