Acquisitions - Summary of Merger-Related Expenses (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Summary of merger-related expenses
Salaries and employee benefits	$ 398,900,000	$ 398,675,000	$ 848,762,000	$ 830,460,000
Equipment and net occupancy	73,797,000	75,724,000	148,163,000	149,902,000
Outside data processing and software	44,575,000	42,509,000	88,876,000	85,524,000
Advertising and marketing	16,324,000	22,613,000	32,434,000	44,067,000
Printing, postage and supplies	8,957,000	9,907,000	18,665,000	21,893,000
Other costs of operations	174,616,000	166,679,000	330,874,000	322,812,000
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
Outside data processing and software		352,000		1,067,000
Advertising and marketing		6,327,000		10,522,000
Printing, postage and supplies		545,000		1,482,000
Other costs of operations		4,970,000		16,072,000
Total	$ 0	$ 12,593,000	$ 0	$ 35,755,000