Employee Benefits - Additional Information (Detail) - benefit_plan	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2009
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Age of employees under condition one	55 years
Period of service under condition one	5 years
Age of employees under condition two	50 years
Period of service under condition two	15 years
Minimum period of service at the time of termination hired before 2001	15 years
First Niagara Bank, N.A. [Member]
Defined Benefit Plan Disclosure [Line Items]
Number of defined benefit plans		2