Consolidated Statements of Changes in Stockholders' Equity - (Unaudited) - USD ($) $ in Millions	Total	Preferred Stock	Common Stock, $0.01 par value per share	Additional Paid-In Capital	Retained Earnings	Unallocated ESOP Common Stock	Accumulated Other Comprehensive Loss	Treasury Stock
Beginning 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	114.6				114.6
Total other comprehensive income (loss), net of tax (note 6)	32.2						32.2
Cash dividends on common stock	(65.2)				(65.2)
Cash dividend on preferred stock	(3.5)				(3.5)
Restricted stock and performance-based share awards	1.0			1.0
ESOP common stock committed to be released (note 9)	1.4				(0.4)	1.8
Common stock repurchased and retired upon vesting of restricted stock awards	(1.7)				(1.7)
Stock options exercised	8.5			8.5
Ending Balance at Mar. 31, 2019	6,621.2	244.1	4.7	6,558.8	1,328.6	(128.3)	(224.6)	(1,162.1)
Beginning 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)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	130.4				130.4
Total other comprehensive income (loss), net of tax (note 6)	74.2						74.2
Cash dividends on common stock	(77.3)				(77.3)
Cash dividend on preferred stock	(3.5)				(3.5)
Restricted stock and performance-based share awards	2.4			2.4
Common stock repurchased (note 6)	(304.4)							(304.4)
ESOP common stock committed to be released (note 9)	1.2				(0.6)	1.8
Common stock repurchased and retired upon vesting of restricted stock awards	(1.4)				(1.4)
Stock options exercised	2.6			2.6
Ending Balance at Mar. 31, 2020	$ 7,725.5	$ 244.1	$ 5.3	$ 7,644.4	$ 1,514.5	$ (121.1)	$ (92.7)	$ (1,469.0)