Regulatory Capital Requirements - Actual Capital Amounts and Ratios and FDIC Minimum Capital Adequacy Requirements (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
FRB
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 leverage capital, Actual Amount	$ 973,214	$ 953,768
Tier 1 leverage capital, Actual Ratio	10.34%	10.24%
Tier 1 leverage capital, minimum capital adequacy requirements, Amount	$ 376,484	$ 372,458
Tier 1 leverage capital, minimum capital adequacy requirements, Ratio	4.00%	4.00%
Tier 1 leverage capital, minimum capital adequacy requirements with capital conservation buffer, Amount	$ 376,484	$ 372,458
Tier 1 leverage capital, minimum capital adequacy requirements with capital conservation buffer, Ratio	4.00%	4.00%
Tier 1 leverage capital, To be well-capitalized under prompt corrective action provisions, Amount	$ 470,605	$ 465,573
Tier 1 leverage capital, To be well-capitalized under prompt corrective action provisions, Ratio	5.00%	5.00%
Common equity Tier 1 risk-based capital, Actual Amount	$ 973,214	$ 953,768
Common equity Tier 1 risk-based capital, Actual Ratio	12.74%	12.54%
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement, Amount	$ 343,756	$ 342,277
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement, Ratio	4.50%	4.50%
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement with capital conversation buffer, Amount	$ 534,732	$ 484,893
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement with capital conversation buffer, Ratio	7.00%	6.38%
Common equity Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Amount	$ 496,537	$ 494,400
Common equity Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Ratio	6.50%	6.50%
Tier 1 risk-based capital, Actual Amount	$ 973,214	$ 953,768
Tier 1 risk-based capital, Actual Ratio	12.74%	12.54%
Tier 1 risk-based capital, minimum capital adequacy requirements, Amount	$ 458,342	$ 456,370
Tier 1 risk-based capital, minimum capital adequacy requirements, Ratio	6.00%	6.00%
Tier 1 risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Amount	$ 649,317	$ 598,985
Tier 1 risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Ratio	8.50%	7.88%
Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Amount	$ 611,122	$ 608,493
Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Ratio	8.00%	8.00%
Total risk-based capital, Actual Amount	$ 1,028,879	$ 1,009,475
Total risk-based capital, Actual Ratio	13.47%	13.27%
Total risk-based capital, minimum capital adequacy requirements, Amount	$ 611,122	$ 608,493
Total risk-based capital, minimum capital adequacy requirements, Ratio	8.00%	8.00%
Total risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Amount	$ 802,098	$ 751,108
Total risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Ratio	10.50%	9.88%
Total risk-based capital, to be well-capitalized under prompt corrective action provisions, Amount	$ 763,903	$ 760,616
Total risk-based capital, to be well-capitalized under prompt corrective action provisions, Ratio	10.00%	10.00%
FDIC
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier 1 leverage capital, Actual Amount	$ 923,471	$ 917,659
Tier 1 leverage capital, Actual Ratio	9.81%	9.86%
Tier 1 leverage capital, minimum capital adequacy requirements, Amount	$ 376,449	$ 372,443
Tier 1 leverage capital, minimum capital adequacy requirements, Ratio	4.00%	4.00%
Tier 1 leverage capital, minimum capital adequacy requirements with capital conservation buffer, Amount	$ 376,449	$ 372,443
Tier 1 leverage capital, minimum capital adequacy requirements with capital conservation buffer, Ratio	4.00%	4.00%
Tier 1 leverage capital, To be well-capitalized under prompt corrective action provisions, Amount	$ 470,562	$ 465,553
Tier 1 leverage capital, To be well-capitalized under prompt corrective action provisions, Ratio	5.00%	5.00%
Common equity Tier 1 risk-based capital, Actual Amount	$ 923,471	$ 917,659
Common equity Tier 1 risk-based capital, Actual Ratio	12.09%	12.06%
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement, Amount	$ 343,716	$ 342,279
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement, Ratio	4.50%	4.50%
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement with capital conversation buffer, Amount	$ 534,670	$ 484,895
Common equity Tier 1 risk-based capital, minimum capital adequacy requirement with capital conversation buffer, Ratio	7.00%	6.38%
Common equity Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Amount	$ 496,479	$ 494,403
Common equity Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Ratio	6.50%	6.50%
Tier 1 risk-based capital, Actual Amount	$ 923,471	$ 917,659
Tier 1 risk-based capital, Actual Ratio	12.09%	12.06%
Tier 1 risk-based capital, minimum capital adequacy requirements, Amount	$ 458,288	$ 456,372
Tier 1 risk-based capital, minimum capital adequacy requirements, Ratio	6.00%	6.00%
Tier 1 risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Amount	$ 649,242	$ 598,988
Tier 1 risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Ratio	8.50%	7.88%
Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Amount	$ 611,051	$ 608,496
Tier 1 risk-based capital, To be well-capitalized under prompt corrective action provisions, Ratio	8.00%	8.00%
Total risk-based capital, Actual Amount	$ 979,136	$ 973,366
Total risk-based capital, Actual Ratio	12.82%	12.80%
Total risk-based capital, minimum capital adequacy requirements, Amount	$ 611,051	$ 608,496
Total risk-based capital, minimum capital adequacy requirements, Ratio	8.00%	8.00%
Total risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Amount	$ 802,004	$ 751,113
Total risk-based capital, minimum capital adequacy requirements with capital conversation buffer, Ratio	10.50%	9.88%
Total risk-based capital, to be well-capitalized under prompt corrective action provisions, Amount	$ 763,814	$ 760,620
Total risk-based capital, to be well-capitalized under prompt corrective action provisions, Ratio	10.00%	10.00%