Regulatory Matters (Tables)

12 Months Ended

Dec. 31, 2017

Regulatory Capital Requirements [Abstract]

Summary Of Actual Capital Amounts And Ratios For The Company And Its Three Largest Subsidiary Banks

(Dollar amounts in millions)	December 31, 2017				To be well-capitalized
	Amount		Ratio		Amount		Ratio
Transitional Basis Basel III Regulatory Capital Rules
Total capital (to risk-weighted assets)
The Company	$	7,628	14.8	%	$	5,146	10.0	%
ZB, National Association	7,306		14.2		5,130		10.0
Tier 1 capital (to risk-weighted assets)
The Company	6,805		13.2		4,116		8.0
ZB, National Association	6,730		13.1		4,104		8.0
Common equity tier 1 capital (to risk-weighted assets)
The Company	6,239		12.1		3,345		6.5
ZB, National Association	5,899		11.5		3,334		6.5
Tier 1 capital (to average assets)
The Company	6,805		10.5		na		na 1
ZB, National Association	6,730		10.4		3,227		5.0

	December 31, 2016				To be well-capitalized
(Dollar amounts in millions)	Amount		Ratio		Amount		Ratio
Transitional Basis Basel III Regulatory Capital Rules
Total capital (to risk-weighted assets)
The Company	$	7,609	15.2	%	$	4,994	10.0	%
ZB, National Association	7,278		14.6		4,983		10.0
Tier 1 capital (to risk-weighted assets)
The Company	6,738		13.5		3,995		8.0
ZB, National Association	6,655		13.4		3,986		8.0
Common equity tier 1 capital (to risk-weighted assets)
The Company	6,028		12.1		3,246		6.5
ZB, National Association	5,824		11.7		3,239		6.5
Tier 1 capital (to average assets)
The Company	6,738		11.1		na		na 1
ZB, National Association	6,655		11.0		3,027		5.0

¹  There is no Tier 1 leverage ratio component in the definition of a well-capitalized bank holding company.