Income Taxes Schedule of Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets:
Reserves not currently deductible for tax	$ 44.6	$ 36.8
Tax credit and loss carryovers	25.4	21.5
Compensation and benefits	24.7	23.1
Lease liability	19.7	35.1
Other	12.6	22.4
Gross deferred tax assets before valuation allowance	127.0	138.9
Valuation allowance	(3.5)	(4.4)
Net deferred tax assets	123.5	134.5
Liabilities:
Property	(1,233.8)	(1,181.3)
Investments	(54.9)	(52.4)
Other	(20.2)	(28.8)
Gross deferred tax liabilities	(1,308.9)	(1,262.5)
Net deferred tax liability	$ (1,185.4)	$ (1,128.0)