Allowance for Credit Losses - Summary of Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Jun. 30, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2013
Receivables [Abstract]
Allowance for non-covered loans	$ 104,351		$ 102,153
Allowance for covered loans	200		200
Total allowance for loan losses	104,551	$ 102,835	102,353	$ 102,438	$ 103,053	$ 113,617
Allowance for unfunded letters of credit	2,146		1,934
Total allowance for credit losses	$ 106,697		$ 104,287