Acquisition 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 ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Business Acquisition [Line Items]
Proceeds from sales of investment securities				$ 63,513,000	$ 5,654,850,000	$ 16,000
Goodwill				4,593,112,000	4,593,112,000
Provision for credit losses				$ 190,000,000	170,000,000	124,000,000
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
Hudson City [Member]
Business Acquisition [Line Items]
Number of banking offices acquired in New Jersey, Connecticut and New York | Office	135
Gross amount of cash paid in the acquisition	$ 2,064,284,000
Business acquisition, purchase price	5,203,108,000
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
Core deposit intangible	$ 131,665,000
Amortization period of core deposit intangible				7 years
Loans acquired				$ 688,000,000
Discount recorded on purchased impaired loans				1,000,000,000
Estimated fair value of purchased impaired loans				18,300,000,000
Outstanding principal of purchased impaired loans				18,000,000,000
Other- than-temporary impairment					102,000,000	104,000,000
Merger-related expenses				$ 35,755,000	96,976,000	$ 0
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