Goodwill Silver Spring Acquisition (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2018	Jan. 05, 2018
Business Acquisition [Line Items]
Business Acquisition, Percentage of Voting Interests Acquired				100.00%
Payments to acquire businesses, gross		$ 802,500
Measurement period adjustments to goodwill acquired	$ (5,978)
Deferred Tax Liabilities, Goodwill	2,000
SIlver Spring Networks, Inc. [Member]
Business Acquisition [Line Items]
Purchase price, net of cash				$ 809,200
Cash acquired from acquisition			$ 97,800
Equity interest issued				$ 6,700
Measurement period adjustments to goodwill acquired	$ (8,000)