Acquisitions - Summary of Merger-Related Expenses (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Summary of merger-related expenses
Salaries and employee benefits	$ 398,675,000	$ 361,657,000	$ 830,460,000	$ 751,550,000
Equipment and net occupancy	75,724,000	66,852,000	149,902,000	133,322,000
Printing, postage and supplies	9,907,000	9,305,000	21,893,000	18,895,000
Other costs of operations	231,801,000	242,048,000	452,403,000	445,017,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits	60,000		5,334,000
Equipment and net occupancy	339,000		1,278,000
Printing, postage and supplies	545,000		1,482,000
Other costs of operations	11,649,000		27,661,000
Total merger-related expenses	$ 12,593,000	$ 0	$ 35,755,000	$ 0