Allowance for Credit Losses - Summary of Allowance for Credit Losses (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Receivables [Abstract]
Allowance for non-covered loans	$ 102,431	$ 102,153
Allowance for covered loans	200	200
Total allowance for loan losses	102,631	102,353	106,709	113,617
Allowance for unfunded letters of credit	1,934	1,934
Total allowance for credit losses	$ 104,565	$ 104,287