Allowance for Expected Credit Losses and Credit Quality - Schedule of Allowance for Loan Losses for Covered and Non-Covered Loan Portfolios (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	$ 146,588	$ 140,571
Provision for loan and lease losses	66,431	12,612
Transfer of balance to OREO and other	0	(2,885)
Charge-offs	(12,119)	(8,918)
Recoveries	2,591	1,586
Allowance for loan and lease losses at end of period	286,685	142,966
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	16,637	14,830
Provision for unfunded lending commitments	2,540	1,151
Reserve for unfunded commitments at end of period	18,302	15,981
Allowance for expected credit losses at end of period	304,987	158,947
Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	83,194	0
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	(875)	0
Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	229,782	140,571
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	15,762	14,830
Real estate - construction | Commercial Loans
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	5,983	4,743
Provision for loan and lease losses	940	936
Transfer of balance to OREO and other		0
Charge-offs	(14)	0
Recoveries	219	3
Allowance for loan and lease losses at end of period	9,435	5,682
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	4,010	3,249
Provision for unfunded lending commitments	492	333
Reserve for unfunded commitments at end of period	5,527	3,582
Allowance for expected credit losses at end of period	14,962	9,264
Real estate - construction | Commercial Loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	2,307
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	1,025
Real estate - construction | Commercial Loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	8,290
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	5,035
Real estate - owner-occupied | Commercial Loans
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	15,770	12,549
Provision for loan and lease losses	2,603	1,303
Transfer of balance to OREO and other		0
Charge-offs	0	(72)
Recoveries	25	35
Allowance for loan and lease losses at end of period	10,235	13,815
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	345	316
Provision for unfunded lending commitments	418	60
Reserve for unfunded commitments at end of period	1,390	376
Allowance for expected credit losses at end of period	11,625	14,191
Real estate - owner-occupied | Commercial Loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	(8,163)
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	627
Real estate - owner-occupied | Commercial Loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	7,607
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	972
Real estate - non-owner-occupied | Commercial Loans
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	36,541	34,514
Provision for loan and lease losses	30,500	4,648
Transfer of balance to OREO and other		0
Charge-offs	(240)	0
Recoveries	132	65
Allowance for loan and lease losses at end of period	96,783	39,227
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	1,629	1,304
Provision for unfunded lending commitments	91	401
Reserve for unfunded commitments at end of period	729	1,705
Allowance for expected credit losses at end of period	97,512	40,932
Real estate - non-owner-occupied | Commercial Loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	29,850
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	(991)
Real estate - non-owner-occupied | Commercial Loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	66,391
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	638
Commercial and industrial | Commercial Loans
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	55,634	54,096
Provision for loan and lease losses	11,833	2,876
Transfer of balance to OREO and other		0
Charge-offs	(7,895)	(4,931)
Recoveries	1,037	446
Allowance for loan and lease losses at end of period	45,950	52,487
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	6,478	6,198
Provision for unfunded lending commitments	925	128
Reserve for unfunded commitments at end of period	7,406	6,326
Allowance for expected credit losses at end of period	53,356	58,813
Commercial and industrial | Commercial Loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	(14,659)
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	3
Commercial and industrial | Commercial Loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	40,975
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	6,481
Residential mortgage | Residential mortgage
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	9,647	12,998
Provision for loan and lease losses	10,535	1,749
Transfer of balance to OREO and other		(2,881)
Charge-offs	(450)	(28)
Recoveries	329	32
Allowance for loan and lease losses at end of period	90,582	11,870
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	562	866
Provision for unfunded lending commitments	215	22
Reserve for unfunded commitments at end of period	1,520	888
Allowance for expected credit losses at end of period	92,102	12,758
Residential mortgage | Residential mortgage | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	70,521
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	743
Residential mortgage | Residential mortgage | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	80,168
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	1,305
Consumer - Home equity | Consumer and other loans
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	12,153	10,181
Provision for loan and lease losses	7,313	1,056
Transfer of balance to OREO and other		(4)
Charge-offs	(1,435)	(1,401)
Recoveries	386	314
Allowance for loan and lease losses at end of period	22,037	10,146
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	2,078	1,783
Provision for unfunded lending commitments	433	28
Reserve for unfunded commitments at end of period	1,699	1,811
Allowance for expected credit losses at end of period	23,736	11,957
Consumer - Home equity | Consumer and other loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	3,620
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	(812)
Consumer - Home equity | Consumer and other loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	15,773
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	1,266
Consumer - Other | Consumer and other loans
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	10,860	11,490
Provision for loan and lease losses	2,707	44
Transfer of balance to OREO and other		0
Charge-offs	(2,085)	(2,486)
Recoveries	463	691
Allowance for loan and lease losses at end of period	11,663	9,739
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	1,535	1,114
Provision for unfunded lending commitments	(34)	179
Reserve for unfunded commitments at end of period	31	1,293
Allowance for expected credit losses at end of period	11,694	$ 11,032
Consumer - Other | Consumer and other loans | Cumulative Effect, Period of Adoption, Adjustment
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	(282)
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	(1,470)
Consumer - Other | Consumer and other loans | Cumulative Effect, Period of Adoption, Adjusted Balance
Allowance for credit losses
Allowance for loan and lease losses at beginning of period	10,578
Reserve for unfunded commitment
Reserve for unfunded commitments at beginning of period	$ 65