Allowance for Credit Losses - Summary of Allowance for Credit Losses (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013
Receivables [Abstract]
Allowance for non-covered loans	$ 102,635		$ 102,153
Allowance for covered loans	200		200
Total allowance for loan losses	102,835	$ 102,631	102,353	$ 103,053	$ 106,709	$ 113,617
Allowance for unfunded letters of credit	2,052		1,934
Total allowance for credit losses	$ 104,887		$ 104,287