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, 2019	Mar. 31, 2018
Purchased Impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 147,210	$ 157,918
Interest income	(18,082)	(9,819)
Reclassifications from nonaccretable balance	11,189	908
Balance at end of period	140,317	149,007
Other Acquired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	96,907	133,162
Interest income	(9,717)	(15,112)
Reclassifications from nonaccretable balance	4,865	207
Other	1,632	(73)
Balance at end of period	$ 93,687	$ 118,184