Acquisitions and Divestiture - Additional Information (Detail)		1 Months Ended		12 Months Ended
	Nov. 01, 2015 USD ($) Office shares	Nov. 30, 2015 USD ($)	Apr. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Proceeds from sales of investment securities				$ 5,654,850,000	$ 16,000	$ 1,081,802,000
Goodwill				4,593,112,000	3,524,625,000
Provision for credit losses				170,000,000	124,000,000	185,000,000
Gain on sale of business net of tax			$ 23,000,000
Gain on sale of business before tax			45,000,000
Goodwill recorded			$ 11,000,000
Revenue on sale of business				9,000,000	34,000,000	38,000,000
Hudson City [Member]
Business Acquisition [Line Items]
Gross amount of cash paid in the acquisition	$ 2,064,284,000
Total purchase consideration	$ 5,203,108,000
Number of banking offices acquired in New Jersey, Connecticut and New York | Office	135
Proceeds from sales of investment securities		$ 5,800,000,000
Repayments of borrowings assumed in acquisition		$ 10,600,000,000
Goodwill	$ 1,079,787,000			1,068,487,000
Core deposit intangible assets				$ 132,000,000
Amortization period of core deposit intangible assets				7 years
Loans acquired				$ 688,000,000
Estimated fair value of purchased impaired loans				18,300,000,000
Outstanding principal of purchased impaired loans				18,000,000,000
Discount recorded on purchased impaired loans				1,000,000,000
Other- than-temporary impairment				102,000,000	104,000,000
Merger-related expenses				96,976,000	$ 0	$ 12,364,000
Merger-related expenses, unpaid portion				56,000,000
Provision for credit losses				$ 21,000,000
Hudson City [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Shares issued upon acquisition | shares	25,953,950