Consolidated Statements of Comprehensive Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Statement of Comprehensive Income [Abstract]
Net income	$ 520.4	$ 468.1	$ 337.2
Net actuarial gains and losses on pension and other postretirement plans:
Net actuarial gains (losses) arising during the year	12.1	(32.6)	(5.5)
Reclassification adjustment for net actuarial loss included in net income	8.3	8.6	9.3
Net actuarial gains (losses)	20.4	(24.0)	3.8
Prior service credit on pension and other postretirement plans:
Reclassification adjustment for prior service credit included in net income	0.0	(0.3)	(0.8)
Net actuarial gains (losses) and prior service credit	20.4	(24.3)	3.0
Net unrealized gains and losses on securities available-for-sale:
Net unrealized holding gains (losses) arising during the year	89.2	(38.3)	(8.6)
Reclassification adjustment for net realized (gains) losses included in net income	(0.1)	9.9	25.4
Net unrealized gains (losses)	89.1	(28.4)	16.8
Net unrealized gains and losses on securities transferred to held-to-maturity:
Reclassification adjustment for amortization of unrealized losses on securities transferred to held-to-maturity included in net income	4.6	3.9	3.7
Net unrealized gains	4.6	3.9	3.7
Net unrealized gains and losses on derivatives accounted for as cash flow hedges:
Net unrealized gains (losses) arising during the year	1.9
Reclassification adjustment for net realized losses (gains) included in net income	1.1
Net unrealized gains (losses)	3.0
Net unrealized gains (losses) arising during the year		(1.7)	(1.0)
Reclassification adjustment for net realized losses (gains) included in net income		0.5	(0.9)
Net unrealized gains (losses)		(1.2)	(1.9)
Other comprehensive income (loss), before tax	117.1	(50.0)	21.6
Deferred income tax (expense) benefit related to other comprehensive income (loss)	(27.2)	12.2	(8.3)
Total other comprehensive income (loss), net of tax	89.9	(37.8)	13.3
Total comprehensive income	$ 610.3	$ 430.3	$ 350.5