Acquisition - Summary of Merger-Related Expenses (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Summary of merger-related expenses
Salaries and employee benefits	$ 431,785,000	$ 389,893,000
Equipment and net occupancy	74,178,000	66,470,000
Printing, postage and supplies	11,986,000	9,590,000
Other cost of operations	220,602,000	202,969,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits	5,274,000
Equipment and net occupancy	939,000
Printing, postage and supplies	937,000
Other cost of operations	16,012,000
Total merger-related expenses	$ 23,162,000	$ 0