New Accounting Standards (Details) - USD ($) $ in Millions		3 Months Ended
	Jan. 01, 2020	Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan losses		$ 341.7	$ 246.6	$ 240.9	$ 240.4
Off-Balance Sheet, Credit Loss, Liability		20.7	5.6
Debt securities, allowance for credit loss		1.9
Retained earnings		$ 1,514.5	1,512.8
Cumulative Effect, Period Of Adoption, Adjustment
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan losses			72.2
Off-Balance Sheet, Credit Loss, Liability			$ 14.5
Accounting Standards Update 2016-13
Summary Of Significant Accounting Policies [Line Items]
Decrease in risk-based capital ratios		0.10%
Accounting Standards Update 2016-13 | Cumulative Effect, Period Of Adoption, Adjustment
Summary Of Significant Accounting Policies [Line Items]
Allowance for loan losses	$ 72.0
Adjustment to amortized cost basis	28.0
Off-Balance Sheet, Credit Loss, Liability	15.0
Debt securities, allowance for credit loss	2.0
Accounting Standards Update 2016-13 | Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment
Summary Of Significant Accounting Policies [Line Items]
Retained earnings	$ (46.0)