Acquisitions and Divestiture - Summary of Merger-Related Expenses (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Summary of merger-related expenses
Salaries and employee benefits	$ 1,549,530,000	$ 1,404,950,000	$ 1,355,178,000
Equipment and net occupancy	272,539,000	269,299,000	264,327,000
Printing, postage and supplies	38,491,000	38,201,000	39,557,000
Other cost of operations	883,835,000	887,669,000	812,308,000
Other expense	2,822,932,000	2,689,474,000	2,587,866,000
Provision for credit losses	170,000,000	124,000,000	185,000,000
Hudson City [Member]
Summary of merger-related expenses
Salaries and employee benefits	51,287,000		836,000
Equipment and net occupancy	3,000		690,000
Printing, postage and supplies	504,000		1,825,000
Other cost of operations	24,182,000		9,013,000
Other expense	75,976,000		12,364,000
Provision for credit losses	21,000,000
Total merger-related expenses	$ 96,976,000	$ 0	$ 12,364,000