Regulatory Matters (Summary Of Actual Capital Amounts And Ratios For The Company And Its Three Largest Subsidiary Banks) (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Total capital (to risk-weighted assets), Actual Amount	$ 7,450
Capital to Risk Weighted Assets	13.90%
Total capital (to risk-weighted assets), To be well capitalized, Amount	$ 5,359
Total capital (to risk-weighted assets), To be well capitalized, Ratio	10.00%
Tier 1 capital (to risk-weighted assets), Actual Amount	$ 6,811
Tier One Leverage Capital to Average Assets	10.30%
Tier One Capital Required to be Well Capitalized	$ 3,317
Tier 1 capital (to risk-weighted assets), Actual Ratio	12.70%
Tier 1 capital (to risk-weighted assets), To be well capitalized, Amount	$ 4,287
Tier 1 capital (to risk-weighted assets), To be well capitalized, Ratio	8.00%
Common Equity Tier One Capital	$ 6,245
Common Equity Tier One Capital Ratio	11.70%
Common Equity Tier One Capital to be Well Capitalized	$ 3,483
Common Equity Tier One Capital to be Well Capitalized Ratio	6.50%
Tier One Capital	$ 6,811
Tier One Capital Required to be Well Capitalized to Average Assets	5.00%
Parent Company [Member]
Total capital (to risk-weighted assets), Actual Amount		$ 7,628
Capital to Risk Weighted Assets		14.80%
Total capital (to risk-weighted assets), To be well capitalized, Amount		$ 5,146
Total capital (to risk-weighted assets), To be well capitalized, Ratio		10.00%
Tier 1 capital (to risk-weighted assets), Actual Amount		$ 6,805
Tier One Leverage Capital to Average Assets		10.50%
Tier 1 capital (to risk-weighted assets), Actual Ratio		13.20%
Tier 1 capital (to risk-weighted assets), To be well capitalized, Amount		$ 4,116
Tier 1 capital (to risk-weighted assets), To be well capitalized, Ratio		8.00%
Common Equity Tier One Capital		$ 6,239
Common Equity Tier One Capital Ratio		12.10%
Common Equity Tier One Capital to be Well Capitalized		$ 3,345
Common Equity Tier One Capital to be Well Capitalized Ratio		6.50%
Tier One Capital		$ 6,805
ZB National Association [Member]
Total capital (to risk-weighted assets), Actual Amount		$ 7,306
Capital to Risk Weighted Assets		14.20%
Total capital (to risk-weighted assets), To be well capitalized, Amount		$ 5,130
Total capital (to risk-weighted assets), To be well capitalized, Ratio		10.00%
Tier 1 capital (to risk-weighted assets), Actual Amount		$ 6,730
Tier One Leverage Capital to Average Assets		10.40%
Tier One Capital Required to be Well Capitalized		$ 3,227
Tier 1 capital (to risk-weighted assets), Actual Ratio		13.10%
Tier 1 capital (to risk-weighted assets), To be well capitalized, Amount		$ 4,104
Tier 1 capital (to risk-weighted assets), To be well capitalized, Ratio		8.00%
Common Equity Tier One Capital		$ 5,899
Common Equity Tier One Capital Ratio		11.50%
Common Equity Tier One Capital to be Well Capitalized		$ 3,334
Common Equity Tier One Capital to be Well Capitalized Ratio		6.50%
Tier One Capital		$ 6,730
Tier One Capital Required to be Well Capitalized to Average Assets		5.00%