Income Taxes - Schedule of Changes in Amount of Uncertain Tax Positions (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Unrecognized liabilities:
Balance at the beginning of the period	$ 16.8	$ 12.2	$ 12.2	$ 16.8
Change in federal tax rate		(4.9)	0.0	0.0
Lapse of applicable statute of limitations	$ (3.0)	0.0	0.0	(4.6)
Balance at the end of the period		$ 7.3	$ 12.2	$ 12.2