Income Taxes - Schedule of Changes in Amount of Uncertain Tax Positions (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Unrecognized liabilities:
Balance at the beginning of the period	$ 7.3	$ 12.2	$ 12.2
Change in federal tax rate	0.0	(4.9)	0.0
Lapse of applicable statute of limitations	0.0	0.0	0.0
Balance at the end of the period	$ 7.3	$ 7.3	$ 12.2