Loans, Leases and Allowance for Credit Losses - Summary of Recorded Investment in Nonaccrual Loans and Loans Past Due 90 Days Still Accruing Interest by Loan Class (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	$ 16,124,002	$ 15,061,371
Total Past Due	14,258	32,564
Total Non-accrual	34,017	43,925
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	43
Commercial real estate [Member] | Owner occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	2,268,882	2,240,457
Total Past Due	1,649	1,156
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Commercial real estate [Member] | Non-owner occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	3,871,249	3,696,729
Total Past Due	0	5,840
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Commercial real estate [Member] | Multi-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	139,327	201,442
Total Past Due	0	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Commercial and industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	7,275,474	6,835,385
Total Past Due	2,934	5,996
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	43
Construction and land development [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	1,301,199	1,090,176
Total Past Due	0	0
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Construction and land development [Member] | Land [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	677,088	536,917
Total Past Due	0	5,111
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Residential real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	535,914	411,857
Total Past Due	9,342	14,083
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	54,869	48,408
Total Past Due	333	378
Financing Receivable, Recorded Investment, 90 Days Past Due and Still Accruing	0	0
Non-accrual loans [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	21,305	21,909
Total Past Due	12,712	22,016
Total Non-accrual	34,017	43,925
Non-accrual loans [Member] | Commercial real estate [Member] | Owner occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	0	1,089
Total Past Due	1,493	792
Total Non-accrual	1,493	1,881
Non-accrual loans [Member] | Commercial real estate [Member] | Non-owner occupied [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	0	0
Total Past Due	0	5,840
Total Non-accrual	0	5,840
Non-accrual loans [Member] | Commercial real estate [Member] | Multi-family [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	0	0
Total Past Due	0	0
Total Non-accrual	0	0
Non-accrual loans [Member] | Commercial and industrial [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	17,231	17,913
Total Past Due	2,120	4,113
Total Non-accrual	19,351	22,026
Non-accrual loans [Member] | Construction and land development [Member] | Construction [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	0	0
Total Past Due	0	0
Total Non-accrual	0	0
Non-accrual loans [Member] | Construction and land development [Member] | Land [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	0	868
Total Past Due	0	5,111
Total Non-accrual	0	5,979
Non-accrual loans [Member] | Residential real estate [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	4,074	2,039
Total Past Due	8,766	6,078
Total Non-accrual	12,840	8,117
Non-accrual loans [Member] | Consumer [Member]
Financing Receivable, Recorded Investment, Past Due [Line Items]
Current	0	0
Total Past Due	333	82
Total Non-accrual	$ 333	$ 82