Loans and Allowance for Credit Losses - Schedule of Average Balance and Interest Income Recognized on Impaired Loans (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Jun. 30, 2015	Dec. 31, 2015
Receivables [Abstract]
loans that were previously covered by loss share agreements with the FDIC	$ 4.7	$ 4.7	$ 4.2