Loans and Leases and the Allowance for Credit Losses - Summary of Changes in Accretable Yield for Acquired Loans (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Purchased Impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 157,918	$ 154,233
Interest income	(9,819)	(10,925)
Reclassifications from nonaccretable balance	908	146
Balance at end of period	149,007	143,454
Other Acquired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	133,162	201,153
Interest income	(15,112)	(25,518)
Reclassifications from nonaccretable balance	207	3,183
Other	(73)	2,492
Balance at end of period	$ 118,184	$ 181,310