Acquisitions - Summary of Merger-Related Expenses (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Summary of merger-related expenses
Salaries and employee benefits	$ 449,862,000	$ 431,785,000
Equipment and net occupancy	74,366,000	74,178,000
Outside data processing and software	44,301,000	43,015,000
Advertising and marketing	16,110,000	21,454,000
Printing, postage and supplies	9,708,000	11,986,000
Other costs of operations	156,258,000	156,133,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits		5,274,000
Equipment and net occupancy		939,000
Outside data processing and software		715,000
Advertising and marketing		4,195,000
Printing, postage and supplies		937,000
Other costs of operations		11,102,000
Total	$ 0	$ 23,162,000