Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended		12 Months Ended
	Dec. 31, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Taxes [Line Items]
Effective Income Tax Rate Reconciliation, Percent		20.84785%	65.90%
Total gross liability for unrecognized tax benefits	$ 12.0	$ 12.7		$ 12.0
Transition tax expense	28.8
Federal Tax Reform [Member]
Income Taxes [Line Items]
Federal Statutory Income Tax Rate, Historical				35.00%
Tax Cuts and Jobs Act of 2017, Corporate Federal Tax Rate				21.00%
Provisional income tax expense	2.7
Decrease of income tax expense related to remeasurement of certain deferred tax assets and liabilities due to lower tax rate	$ 26.1