Acquisitions - Additional Information (Detail)		1 Months Ended	3 Months Ended		6 Months Ended
	Nov. 01, 2015 USD ($) Office shares	Nov. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Proceeds from sales of investment securities					$ 4,970,000	$ 2,539,000
Goodwill			$ 4,593,112,000		4,593,112,000		$ 4,593,112,000
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
Core deposit intangible assets			132,000,000		$ 132,000,000
Amortization period of core deposit intangible assets					7 years
Other- than-temporary impairment				$ 67,000,000		74,000,000
Merger-related expenses			$ 12,593,000	$ 0	$ 35,755,000	$ 0
Hudson City [Member] | Common Stock [Member]
Business Acquisition [Line Items]
Shares issued upon acquisition | shares	25,953,950