Acquisition and Divestiture - Summary of Merger-Related Expenses (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Summary of merger-related expenses
Salaries and employee benefits	$ 1,623,600,000	$ 1,549,530,000	$ 1,404,950,000
Equipment and net occupancy	295,141,000	272,539,000	269,299,000
Outside data processing and software	172,389,000	164,133,000	151,568,000
Advertising and marketing	87,137,000	59,227,000	47,111,000
Printing, postage and supplies	39,546,000	38,491,000	38,201,000
Other costs of operations	682,014,000	660,475,000	688,990,000
Total other expense	3,047,485,000	2,822,932,000	2,689,474,000
Provision for credit losses	190,000,000	170,000,000	124,000,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits	5,334,000	51,287,000
Equipment and net occupancy	1,278,000	3,000
Outside data processing and software	1,067,000	785,000
Advertising and marketing	10,522,000	79,000
Printing, postage and supplies	1,482,000	504,000
Other costs of operations	16,072,000	23,318,000
Total other expense	35,755,000	75,976,000
Provision for credit losses		21,000,000
Total	$ 35,755,000	$ 96,976,000	$ 0