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		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Purchased Impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 140,317	$ 149,007	$ 147,210	$ 157,918
Interest income	(9,632)	(7,969)	(27,714)	(17,788)
Reclassifications from nonaccretable balance	16,419	8,350	27,608	9,258
Balance at end of period	147,104	149,388	147,104	149,388
Other Acquired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	93,687	118,184	96,907	133,162
Interest income	(9,666)	(15,394)	(19,383)	(30,506)
Reclassifications from nonaccretable balance	3,457	10,998	8,322	11,205
Other	3,433	3,927	5,065	3,854
Balance at end of period	$ 90,911	$ 117,715	$ 90,911	$ 117,715