Regulatory Matters (Summary Of Actual Capital Amounts And Ratios For The Company And Its Three Largest Subsidiary Banks) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Parent Company [Member]
Total capital (to risk-weighted assets), Actual Amount	$ 7,608,502	$ 7,535,760
Capital to Risk Weighted Assets	15.24%	16.12%
Total capital (to risk-weighted assets), To be well capitalized, Amount	$ 4,993,671	$ 4,674,725
Total capital (to risk-weighted assets), To be well capitalized, Ratio	10.00%	10.00%
Tier 1 capital (to risk-weighted assets), Actual Amount	$ 6,737,638	$ 6,580,326
Tier One Leverage Capital to Average Assets	11.09%	1126.00%
Tier 1 capital (to risk-weighted assets), Actual Ratio	13.49%	1408.00%
Tier 1 capital (to risk-weighted assets), To be well capitalized, Amount	$ 3,994,937	$ 3,739,780
Tier 1 capital (to risk-weighted assets), To be well capitalized, Ratio	8.00%	800.00%
Common Equity Tier One Capital	$ 6,028,037	$ 5,711,836
Common Equity Tier One Capital Ratio	12.07%	1222.00%
Common Equity Tier One Capital to be Well Capitalized	$ 3,245,886	$ 3,038,571
Common Equity Tier One Capital to be Well Capitalized Ratio	6.50%	650.00%
Tier One Capital	$ 6,737,638	$ 6,580,326
ZB National Association [Member]
Total capital (to risk-weighted assets), Actual Amount	$ 7,277,987	$ 6,918,312
Capital to Risk Weighted Assets	14.61%	1484.00%
Total capital (to risk-weighted assets), To be well capitalized, Amount	$ 4,983,000	$ 4,661,581
Total capital (to risk-weighted assets), To be well capitalized, Ratio	10.00%	1000.00%
Tier 1 capital (to risk-weighted assets), Actual Amount	$ 6,654,990	$ 6,334,391
Tier One Leverage Capital to Average Assets	10.99%	1097.00%
Tier One Capital Required to be Well Capitalized	$ 3,027,427	$ 2,886,732
Tier 1 capital (to risk-weighted assets), Actual Ratio	13.36%	1359.00%
Tier 1 capital (to risk-weighted assets), To be well capitalized, Amount	$ 3,986,400	$ 3,729,265
Tier 1 capital (to risk-weighted assets), To be well capitalized, Ratio	8.00%	800.00%
Common Equity Tier One Capital	$ 5,824,090	$ 5,503,491
Common Equity Tier One Capital Ratio	11.69%	1181.00%
Common Equity Tier One Capital to be Well Capitalized	$ 3,238,950	$ 3,030,028
Common Equity Tier One Capital to be Well Capitalized Ratio	6.50%	650.00%
Tier One Capital	$ 6,654,990	$ 6,334,391
Tier One Capital Required to be Well Capitalized to Average Assets	5.00%	500.00%