Regulatory Matters (Summary Of Actual Capital Amounts And Ratios For The Company And Its Three Largest Subsidiary Banks) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Parent Company [Member]
Total capital (to risk-weighted assets), Actual Amount	$ 7,628	$ 7,609
Capital to Risk Weighted Assets	14.80%	15.20%
Total capital (to risk-weighted assets), To be well capitalized, Amount	$ 5,146	$ 4,994
Total capital (to risk-weighted assets), To be well capitalized, Ratio	10.00%	10.00%
Tier 1 capital (to risk-weighted assets), Actual Amount	$ 6,805	$ 6,738
Tier One Leverage Capital to Average Assets	10.50%	11.10%
Tier 1 capital (to risk-weighted assets), Actual Ratio	13.20%	13.50%
Tier 1 capital (to risk-weighted assets), To be well capitalized, Amount	$ 4,116	$ 3,995
Tier 1 capital (to risk-weighted assets), To be well capitalized, Ratio	8.00%	8.00%
Common Equity Tier One Capital	$ 6,239	$ 6,028
Common Equity Tier One Capital Ratio	12.10%	12.10%
Common Equity Tier One Capital to be Well Capitalized	$ 3,345	$ 3,246
Common Equity Tier One Capital to be Well Capitalized Ratio	6.50%	6.50%
Tier One Capital	$ 6,805	$ 6,738
ZB National Association [Member]
Total capital (to risk-weighted assets), Actual Amount	$ 7,306	$ 7,278
Capital to Risk Weighted Assets	14.20%	14.60%
Total capital (to risk-weighted assets), To be well capitalized, Amount	$ 5,130	$ 4,983
Total capital (to risk-weighted assets), To be well capitalized, Ratio	10.00%	10.00%
Tier 1 capital (to risk-weighted assets), Actual Amount	$ 6,730	$ 6,655
Tier One Leverage Capital to Average Assets	10.40%	11.00%
Tier One Capital Required to be Well Capitalized	$ 3,227	$ 3,027
Tier 1 capital (to risk-weighted assets), Actual Ratio	13.10%	13.40%
Tier 1 capital (to risk-weighted assets), To be well capitalized, Amount	$ 4,104	$ 3,986
Tier 1 capital (to risk-weighted assets), To be well capitalized, Ratio	8.00%	8.00%
Common Equity Tier One Capital	$ 5,899	$ 5,824
Common Equity Tier One Capital Ratio	11.50%	11.70%
Common Equity Tier One Capital to be Well Capitalized	$ 3,334	$ 3,239
Common Equity Tier One Capital to be Well Capitalized Ratio	6.50%	6.50%
Tier One Capital	$ 6,730	$ 6,655
Tier One Capital Required to be Well Capitalized to Average Assets	5.00%	5.00%