Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Tax Credit Carryforward [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 0.0	$ 0.0	$ 70.1
Significant Change in Unrecognized Tax Benefits is Reasonably Possible, Amount of Unrecorded Benefit	$ 1.1
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	35.00%
Deferred Tax Assets, Valuation Allowance	$ 5.3	$ 3.8
Change in valuation allowance	2.3	0.0	$ 0.8
Unrecognized Tax Benefits, Income Tax Penalties and Interest Accrued	3.4	2.8
Unrecognized tax benefits , if recognized, would favorably impact effective tax rate	3.5	2.8	3.6
Accrued interest related to tax obligations	0.1	0.1	0.1
Federal and state credit carryforwards	10.3	10.7
Investment Tax Credit	1.3	10.0
Income Tax Examination, Penalties Accrued	0.0		0.0	$ 0.0
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Impairment Losses, Amount	$ 0.0	$ 41.0	$ 0.0
Deferred Tax Assets, Valuation Allowance Expirations [Domain]
Tax Credit Carryforward [Line Items]
Valuation Allowance, Deferred Tax Asset, Explanation of Change	.8
Valuation allowances, State Tax Credits [Member]
Tax Credit Carryforward [Line Items]
Change in valuation allowance	$ (7.7)