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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Purchased Impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 149,388	$ 133,532	$ 157,918	$ 154,233
Interest income	(8,105)	(10,815)	(25,893)	(32,546)
Reclassifications from nonaccretable balance	8,445	30,799	17,703	31,829
Balance at end of period	149,728	153,516	149,728	153,516
Other Acquired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	117,715	163,099	133,162	201,153
Interest income	(18,001)	(20,064)	(48,507)	(66,505)
Reclassifications from nonaccretable balance	25	6,041	11,230	11,076
Other	2,001	1,545	5,855	4,897
Balance at end of period	$ 101,740	$ 150,621	$ 101,740	$ 150,621