Selected Quarterly Financial Data (unaudited) - Quarterly Financial Data (Footnote) (Detail) - USD ($) $ in Millions	1 Months Ended	3 Months Ended								9 Months Ended	12 Months Ended				50 Months Ended
	Jul. 31, 2016	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2015	Dec. 31, 2016
Income Statement [Abstract]
Prior Period Reclassification Adjustment	$ 0.6				$ 6.5	$ 0.7	$ 0.7		$ 2.5		$ 2.6	$ 0.7	$ 6.3	$ 2.4	$ 0.6
prior year reclassification of amortization		$ 1.8	$ 1.7	$ 1.6	$ 1.7	$ 1.6	$ 1.6	$ 1.6		$ 0.7