Acquisition and Divestiture - Additional Information (Detail) $ in Thousands		1 Months Ended	12 Months Ended
	Nov. 01, 2015 USD ($) Office shares	Apr. 30, 2015 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Proceeds from sales of investment securities Available for sale			$ 534,160	$ 63,513	$ 5,654,850
Goodwill			4,593,112	4,593,112
Provision for credit losses			168,000	190,000	170,000
Gain on sale of business net of tax		$ 23,000
Gain on sale of business before tax		45,000
Goodwill recorded		$ 11,000
Revenue on sale of business					9,000
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	$ 2,100,000
Business acquisition, purchase price	5,200,000
Assets acquired	36,700,000
Residential real estate loans	19,000,000
Investment securities	7,900,000
Liabilities assumed	31,500,000
Deposits	17,900,000
Borrowings	13,200,000
Proceeds from sales of investment securities Available for sale	5,800,000
Repayments of borrowings	10,600,000
Goodwill	1,100,000
Core deposit intangible asset	$ 132,000
Estimated useful lives	7 years
Merger-related expenses			$ 0	35,755	96,976
Provision for credit losses					$ 21,000
Estimated fair value of purchased impaired loans				$ 18,300,000
Hudson City [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Shares issued upon acquisition | shares	25,953,950