Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Millions	Total	Suffolk [Member]	First Connecticut Bancorp, Inc. [Member]	BSB Bancorp [Member]	United Financial [Member]	Preferred Stock [Member]	Common Stock [Member]	Common Stock [Member] Suffolk [Member]	Common Stock [Member] First Connecticut Bancorp, Inc. [Member]	Common Stock [Member] BSB Bancorp [Member]	Common Stock [Member] United Financial [Member]	Additional Paid-In Capital [Member]	Additional Paid-In Capital [Member] Suffolk [Member]	Additional Paid-In Capital [Member] First Connecticut Bancorp, Inc. [Member]	Additional Paid-In Capital [Member] BSB Bancorp [Member]	Additional Paid-In Capital [Member] United Financial [Member]	Retained Earnings [Member]	Unallocated ESOP Common Stock [Member]	[1]	Accumulated Other Comprehensive Loss [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2016	$ 5,141.9					$ 244.1	$ 4.0					$ 5,446.1					$ 949.3	$ (144.6)		$ (195.0)	$ (1,162.0)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	337.2																337.2
Total other comprehensive income, net of tax (note 17)	13.3																			13.3
Common stock issued in acquisitions	484.8	$ 484.8						$ 0.2					$ 484.6
Cash dividends on common stock	(227.9)																(227.9)
Cash dividends on preferred stock	(14.1)																(14.1)
Restricted stock and performance- based share awards	14.2											14.3									(0.1)
ESOP common stock committed to be released (note 18)	6.4																(0.9)	7.3
Common stock repurchased and retired upon vesting of restricted stock awards (note 19)	(3.4)																(3.4)
Stock options and related tax benefits	67.5						0.2					67.3
Ending Balance at Dec. 31, 2017	5,819.9					244.1	4.4					6,012.3					1,040.2	(137.3)		(181.7)	(1,162.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	468.1																468.1
Total other comprehensive income, net of tax (note 17)	(37.8)																			(37.8)
Common stock issued in acquisitions	486.4		$ 486.4						$ 0.3					$ 486.1
Cash dividends on common stock	(243.8)																(243.8)
Cash dividends on preferred stock	(14.1)																(14.1)
Restricted stock and performance- based share awards	17.9											17.9									0.0
ESOP common stock committed to be released (note 18)	6.2																(1.0)	7.2
Common stock repurchased and retired upon vesting of restricted stock awards (note 19)	(2.5)																(2.5)
Stock options exercised	33.0						0.0					33.0
Ending Balance at Dec. 31, 2018	6,533.9					244.1	4.7					6,549.3					1,284.8	(130.1)		(256.8)	(1,162.1)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	520.4																520.4
Total other comprehensive income, net of tax (note 17)	89.9																			89.9
Common stock issued in acquisitions	1,045.1			$ 324.5	$ 720.6					$ 0.2	$ 0.4				$ 324.3	$ 720.2
Cash dividends on common stock	(274.8)																(274.8)
Cash dividends on preferred stock	(14.1)																(14.1)
Restricted stock and performance- based share awards	16.3											16.3
Common stock repurchased (note 14)	(2.5)																				(2.5)
ESOP common stock committed to be released (note 18)	5.7																(1.5)	7.2
Common stock repurchased and retired upon vesting of restricted stock awards (note 19)	(2.0)																(2.0)
Stock options exercised	29.3											29.3
Ending Balance at Dec. 31, 2019	$ 7,947.2					$ 244.1	$ 5.3					$ 7,639.4					$ 1,512.8	$ (122.9)		$ (166.9)	$ (1,164.6)

[1] Employee Stock Ownership Plan