Summary of Significant Accounting Policies - Contract Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Contract Assets
Balance at beginning of period	$ 30,116
Change due to acquisitions	2,833
Change due to conditional versus unconditional	6,244
Reclassified to unbilled accounts receivable	(28,980)
Balance at end of period	10,213	$ 30,116
Contract Liabilities
Balance at beginning of period	106,005
Change due to acquisitions	8,195
Change in timing for performance obligation to be satisfied	16,786
Balance at end of period	130,986	106,005
Revenue related to our contract liabilities	$ 97,600
The term of the renewable service maintenance agreements (in years)	1 year
Practical Expedient	true
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Contract Liabilities
Revenue related to our contract liabilities		$ 86,600

Amount of increase (decrease) in contract with customer assets due to change in conditional versus unconditional.

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The term of renewable service maintenance agreements in years.

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Amount of increase (decrease) in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, from business combination.

+ References

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Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

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Amount of decrease in right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time from transfer to receivable due to right to consideration becoming unconditional.

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Amount of revenue recognized arising from contract liability from change in timeframe for performance obligation to be satisfied.

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Amount of obligation to transfer good or service to customer for which consideration has been received or is receivable, classified as current.

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Amount of increase (decrease) in obligation to transfer good or service to customer for which consideration from customer has been received or is due, from business combination.

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Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

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Indicates (true false) whether optional exemption was applied not to disclose amount of transaction price allocated to and explanation of expected timing of revenue recognition for remaining performance obligation, when contract has expected duration of one year or less or right to consideration corresponds directly to performance completed.

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