Loans and Allowance for Loan Losses - Schedule of Risk Category of Commercial and Commercial Real Estate Loans (Detail) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Risk Category Of Loans [Line Items]
Total	$ 12,017,648	$ 12,243,892	$ 11,292,659
Commercial Loan [Member]
Risk Category Of Loans [Line Items]
Total	2,950,559	3,232,970	2,949,277
Commercial Real Estate [Member]
Risk Category Of Loans [Line Items]
Total	5,112,123	4,958,851	$ 4,481,554
Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	592,348	504,625
Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	4,519,775	4,454,226
Pass [Member] | Commercial Loan [Member]
Risk Category Of Loans [Line Items]
Total	2,773,941	3,029,130
Pass [Member] | Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	553,854	460,158
Pass [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	4,251,014	4,167,902
Criticized [Member] | Commercial Loan [Member]
Risk Category Of Loans [Line Items]
Total	89,079	98,798
Criticized [Member] | Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	24,839	29,368
Criticized [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	119,601	110,586
Classified - Substandard [Member] | Commercial Loan [Member]
Risk Category Of Loans [Line Items]
Total	49,659	66,394
Classified - Substandard [Member] | Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total		1,275
Classified - Substandard [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	92,372	102,961
Classified - Nonaccrual [Member] | Commercial Loan [Member]
Risk Category Of Loans [Line Items]
Total	30,067	29,003
Classified - Nonaccrual [Member] | Commercial Real Estate [Member] | Construction [Member]
Risk Category Of Loans [Line Items]
Total	13,655	13,824
Classified - Nonaccrual [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	37,972	37,441
Classified - Doubtful [Member] | Commercial Loan [Member]
Risk Category Of Loans [Line Items]
Total	7,813	9,645
Classified - Doubtful [Member] | Commercial Real Estate [Member] | Other [Member]
Risk Category Of Loans [Line Items]
Total	$ 18,816	$ 35,336