Loans and the Allowance for Credit Losses - Summary of Commercial Loans by Risk Grade (Detail) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Summary of commercial loans by risk grade
Commercial loans	$ 7,686,995	$ 7,607,333
Pass [Member]
Summary of commercial loans by risk grade
Commercial loans	5,063,970	5,036,201
Criticized [Member]
Summary of commercial loans by risk grade
Commercial loans	73,236	51,710
Classified - substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	41,004	31,244
Commercial Real Estate - Land and Construction [Member]
Summary of commercial loans by risk grade
Commercial loans	483,046	528,072
Commercial Real Estate - Land and Construction [Member] | Pass [Member]
Summary of commercial loans by risk grade
Commercial loans	476,109	523,707
Commercial Real Estate - Land and Construction [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Commercial loans	5,794	2,297
Commercial Real Estate - Land and Construction [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	1,143	2,068
Commercial Real Estate - Improved Property [Member]
Summary of commercial loans by risk grade
Commercial loans	3,394,587	3,325,623
Commercial Real Estate - Improved Property [Member] | Pass [Member]
Summary of commercial loans by risk grade
Commercial loans	3,319,270	3,267,304
Commercial Real Estate - Improved Property [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Commercial loans	52,657	35,566
Commercial Real Estate - Improved Property [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	22,660	22,753
Commercial and Industrial [Member]
Summary of commercial loans by risk grade
Commercial loans	1,300,577	1,265,460
Commercial and Industrial [Member] | Pass [Member]
Summary of commercial loans by risk grade
Commercial loans	1,268,591	1,245,190
Commercial and Industrial [Member] | Criticized [Member]
Summary of commercial loans by risk grade
Commercial loans	14,785	13,847
Commercial and Industrial [Member] | Classified - substandard [Member]
Summary of commercial loans by risk grade
Commercial loans	17,201	6,423
Commercial Portfolio Segment [Member]
Summary of commercial loans by risk grade
Commercial loans	$ 5,178,210	$ 5,119,155