Capital - Actual Capital Amounts and Ratios for Community Bank in Comparison to Minimum Amounts and Ratios Required (Detail) - Company - USD ($) $ in Millions	Dec. 31, 2021	Dec. 31, 2020
Common Equity, Tier 1 Amount
Total regulatory capital, Common Equity, Tier 1 Amount	$ 4,226	$ 3,962
Minimum for capital adequacy purposes, Common Equity, Tier 1 Amount	1,966	1,834
Excess, Common Equity, Tier 1 Amount	$ 2,260	$ 2,128
Common Equity, Tier 1 Ratio
Total regulatory capital, Common Equity, Tier 1 Ratio	9.68%	9.72%
Minimum for capital adequacy purposes, Common Equity, Tier 1 Ratio	4.50%	4.50%
Excess, Common Equity, Tier 1 Ratio	5.18%	5.22%
Risk-Based Capital, Tier 1 Amount
Total regulatory capital, Risk-Based Capital, Tier 1 Amount	$ 4,729	$ 4,465
Minimum for capital adequacy purposes, Risk-Based Capital, Tier 1 Amount	2,621	2,446
Excess, Risk-Based Capital, Tier 1 Amount	$ 2,108	$ 2,019
Risk-Based Capital, Tier 1 Ratio
Total regulatory capital, Risk-Based Capital, Tier 1 Ratio	10.83	10.95
Minimum for capital adequacy purposes, Risk-Based Capital, Tier 1 Ratio	6.00	6.00
Excess, Risk-Based Capital, Tier 1 Ratio	4.83	4.95
Risk-Based Capital, Total Amount
Total regulatory capital, Risk-Based Capital, Total Amount	$ 5,558	$ 5,290
Minimum for capital adequacy purposes, Risk-Based Capital, Total Amount	3,494	3,262
Excess, Risk-Based Capital, Total Amount	$ 2,064	$ 2,028
Risk-Based Capital, Total Ratio
Total regulatory capital, Risk-Based Capital, Total Ratio	12.73	12.97
Minimum for capital adequacy purposes, Risk-Based Capital, Total Ratio	8.00	8.00
Excess, Risk-Based Capital, Total Ratio	4.73	4.97
Leverage Capital, Amount
Total regulatory capital, Leverage Capital, Amount	$ 4,729	$ 4,465
Minimum for capital adequacy purposes, Leverage Capital, Amount	2,237	2,096
Excess, Leverage Capital, Amount	$ 2,492	$ 2,369
Leverage Capital, Ratio
Total regulatory capital, Leverage Capital, Ratio	8.46	8.52
Minimum for capital adequacy purposes, Leverage Capital, Ratio	4.00	4.00
Excess, Leverage Capital, Ratio	4.46	4.52