Consolidated Statements of Comprehensive Income - (Unaudited) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Statement of Comprehensive Income [Abstract]
Net income	$ 130.4	$ 114.6
Other comprehensive income, net of tax:
Net actuarial gains and losses related to pension and other postretirement plans	1.6	1.8
Net unrealized gains and losses on debt securities available-for-sale	71.6	29.0
Amortization of unrealized losses on debt securities transferred to held-to-maturity	1.0	0.6
Net unrealized gains and losses on derivatives accounted for as cash flow hedges	0.0	0.8
Total other comprehensive income, net of tax (note 6)	74.2	32.2
Total comprehensive income	$ 204.6	$ 146.8