Other Noncurrent Assets - Additional Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other Assets [Abstract]
Operating lease expense	$ 6.2
Operating lease, right-of-use asset	1.4
Operating lease, liability	$ 1.4
Weighted average remaining lease term	13 years
Weighted average discount rate associated with leases	4.60%
Net operating lease rent expense		$ 14.8	$ 16.3	$ 14.3