Acquisition and Divestiture - Summary of Merger-Related Expenses (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Summary of merger-related expenses
Salaries and employee benefits	$ 1,650,729	$ 1,623,600	$ 1,549,530
Equipment and net occupancy	295,084	295,141	272,539
Outside data processing and software	184,670	172,389	164,133
Advertising and marketing	69,203	87,137	59,227
Printing, postage and supplies	35,960	39,546	38,491
Other costs of operations	771,442	682,014	660,475
Total other expense	3,140,325	3,047,485	2,822,932
Provision for credit losses	168,000	190,000	170,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits		5,334	51,287
Equipment and net occupancy		1,278	3
Outside data processing and software		1,067	785
Advertising and marketing		10,522	79
Printing, postage and supplies		1,482	504
Other costs of operations		16,072	23,318
Total other expense		35,755	75,976
Provision for credit losses			21,000
Total	$ 0	$ 35,755	$ 96,976