Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Deferred tax assets classified as other, tax credit carryforwards	$ 6.1
Statutory federal income tax rate	35.00%	35.00%	35.00%
Decrease in liability for unrecognized tax benefits within the next twelve months	$ 4.2
Uncertain tax position, accrued interest	1.8	$ 4.6
Federal
Income Taxes [Line Items]
Operating loss carryforwards	71.6
Obligations of states and state agencies
Income Taxes [Line Items]
Operating loss carryforwards	405.0
Tax credit carryforward	$ 2.9