Acquisitions - Summary of Merger-Related Expenses (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Summary of merger-related expenses
Salaries and employee benefits	$ 1,404,950,000	$ 1,355,178,000	$ 1,314,540,000
Equipment and net occupancy	269,299,000	264,327,000	257,551,000
Printing, postage and supplies	38,201,000	39,557,000	41,929,000
Other costs of operations	941,052,000	860,327,000	733,499,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits		836,000	4,997,000
Equipment and net occupancy		690,000	15,000
Printing, postage and supplies		1,825,000
Other costs of operations		9,013,000	4,867,000
Total merger-related expenses	$ 0	$ 12,364,000	$ 9,879,000