Loans and the Allowance for Credit Losses - Summary of Commercial Loans by Risk Grade (Detail) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	$ 10,789,233	$ 10,267,985
Commercial and Industrial [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	2,407,438	1,644,699
Commercial and Industrial [Member] | Pass [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	2,350,724
Commercial and Industrial [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	34,597
Commercial and Industrial [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	22,117
Commercial Portfolio Segment [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	8,112,830	7,369,707
Commercial Portfolio Segment [Member] | Pass [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	7,617,885	7,147,229
Commercial Portfolio Segment [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	362,295	118,959
Commercial Portfolio Segment [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	132,650	91,749
Commercial Portfolio Segment [Member] | Classified - doubtful [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income		11,770
Commercial Portfolio Segment [Member] | Commercial Real Estate - Land and Construction [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	668,277	777,151
Commercial Portfolio Segment [Member] | Commercial Real Estate - Land and Construction [Member] | Pass [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	657,435	769,537
Commercial Portfolio Segment [Member] | Commercial Real Estate - Land and Construction [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	7,397	4,338
Commercial Portfolio Segment [Member] | Commercial Real Estate - Land and Construction [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	3,445	3,276
Commercial Portfolio Segment [Member] | Commercial Real Estate - Improved Property [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	5,037,115	4,947,857
Commercial Portfolio Segment [Member] | Commercial Real Estate - Improved Property [Member] | Pass [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	4,609,726	4,807,003
Commercial Portfolio Segment [Member] | Commercial Real Estate - Improved Property [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	320,301	65,612
Commercial Portfolio Segment [Member] | Commercial Real Estate - Improved Property [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	107,088	75,242
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	2,407,438	1,644,699
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member] | Pass [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	2,350,724	1,570,689
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	34,597	49,009
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income	$ 22,117	13,231
Commercial Portfolio Segment [Member] | Commercial and Industrial [Member] | Classified - doubtful [Member]
Summary of commercial loans by risk grade
Portfolio loans, net of unearned income		$ 11,770