Acquisitions - Summary of Merger-Related Expenses (Detail) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Summary of merger-related expenses
Salaries and employee benefits	$ 399,786,000	$ 363,567,000	$ 1,230,246,000	$ 1,115,117,000
Equipment and net occupancy	75,263,000	68,470,000	225,165,000	201,792,000
Printing, postage and supplies	8,972,000	8,691,000	30,865,000	27,586,000
Other costs of operations	230,125,000	197,908,000	682,528,000	642,925,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits			5,334,000
Equipment and net occupancy			1,278,000
Printing, postage and supplies			1,482,000
Other costs of operations			27,661,000
Total	$ 0	$ 0	$ 35,755,000	$ 0