Leases - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Lessee, Lease, Description [Line Items]
Operating and Finance Lease, Option to Extend	5 years
Operating and Finance Lease, Option to Terminate	1 year
Finance lease, right of use asset	$ 9.4	$ 8.7
Operating Leases, Rent Expense		$ 29.4	$ 27.3	$ 25.1
Minimum
Lessee, Lease, Description [Line Items]
Operating And Finance Leases Remaining Lease Term	1 year
Maximum
Lessee, Lease, Description [Line Items]
Operating And Finance Leases Remaining Lease Term	10 years