Borrowed Funds - Additional Information (Detail) - USD ($)		1 Months Ended			3 Months Ended				12 Months Ended
	Jan. 01, 2018	Aug. 31, 2016	Jun. 30, 2015	Sep. 30, 2013	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 01, 2019
Debt Instrument [Line Items]
Weighted average interest rate for short-term borrowings					1.68%				1.68%	2.45%
Prepaid long-term FHLB advances					$ 635,000,000.0				$ 635,000,000.0
Callable FHLB advances interest rate					3.93%				3.93%
Loss on extinguishment of debt					$ (31,995,000)	$ 0	$ 0	$ 0	$ 31,995,000	$ 0	$ 0
FHLB advances callable for early redemption					0				0
Subordinated notes, interest rate	6.25%
Fair value of securities					$ 2,300,000,000				$ 2,300,000,000	$ 2,400,000,000
Interest rate swaps
Debt Instrument [Line Items]
Notional amount of terminated derivative		$ 125,000,000
Securities sold under agreements to repurchase
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings					1.94%				1.94%	3.70%
FHLB
Debt Instrument [Line Items]
Weighted average interest rate for long-term borrowings					2.23%				2.23%	3.13%
Subordinated notes
Debt Instrument [Line Items]
Subordinated notes			$ 100,000,000
Subordinated notes, interest rate			4.55%
Net carrying value of subordinated debentures					$ 99,400,000				$ 99,400,000	$ 99,300,000
Subordinated notes due September 2023
Debt Instrument [Line Items]
Subordinated notes				$ 125,000,000
Subordinated notes, interest rate				5.125%
Net carrying value of subordinated debentures					132,400,000				132,400,000	134,200,000
Effective interest rate		3.32%
Oritani Financial Corp.
Debt Instrument [Line Items]
Assumed FHLB advances												$ 430,130,000
USAmeriBancorp, Inc.
Debt Instrument [Line Items]
Subordinated notes	$ 60,000,000
Net carrying value of subordinated debentures					$ 60,600,000				$ 60,600,000	$ 61,100,000