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, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Purchased Impaired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	$ 149,007	$ 143,454	$ 157,918	$ 154,233
Interest income	(7,969)	(10,806)	(17,788)	(21,731)
Reclassifications from nonaccretable balance	8,350	884	9,258	1,030
Balance at end of period	149,388	133,532	149,388	133,532
Other Acquired [Member]
Summary of changes in Accretable Yield for acquired loans
Balance at beginning of period	118,184	181,310	133,162	201,153
Interest income	(15,394)	(20,923)	(30,506)	(46,441)
Reclassifications from nonaccretable balance	10,998	1,852	11,205	5,035
Other	3,927	860	3,854	3,352
Balance at end of period	$ 117,715	$ 163,099	$ 117,715	$ 163,099