Sale of Business Sale of a business (Notes)	12 Months Ended
Dec. 31, 2018
Sale of business [Abstract]
Sale of business [Text Block]	Sale of Security Services Business On December 5, 2018, the Company completed the sale of the rights, economic benefits, and obligations, in all customer contracts related to its Security Services business, which was primarily comprised of Distributed Denial of Service Protection and Managed DNS services, to Neustar. As part of the transaction, the Company will continue to support the Security Services customers during the transition to Neustar over the course of 2019. The transaction was accounted for as the sale of a business. The Company received a payment of $50.0 million at closing and recorded a non-current receivable for the estimated contingent consideration of $14.7 million expected to be collected after the first anniversary of closing. In addition, the Company recorded a current liability of $9.9 million for amounts due to Neustar for estimated collections from Security Services customers of any billings after the closing date and until the customer contracts are assigned to Neustar. As a result of the sale, the Company recognized a pre-tax gain of approximately $54.8 million, which is included in Non-operating income in the fourth quarter of 2018. The estimated contingent consideration to be received, the liability for estimated future billings to be remitted to Neustar, and the gain recognized in 2018, are based on the Company’s best estimates of the probability and timing of customers consenting to the assignment of their contracts to Neustar. To the extent that the actual results differ from the Company’s estimates, the gain on the sale may be adjusted in 2019.

Sale of Security Services Business

On December 5, 2018, the Company completed the sale of the rights, economic benefits, and obligations, in all customer contracts related to its Security Services business, which was primarily comprised of Distributed Denial of Service Protection and Managed DNS services, to Neustar. As part of the transaction, the Company will continue to support the Security Services customers during the transition to Neustar over the course of 2019. The transaction was accounted for as the sale of a business. The Company received a payment of $50.0 million at closing and recorded a non-current receivable for the estimated contingent consideration of $14.7 million expected to be collected after the first anniversary of closing. In addition, the Company recorded a current liability of $9.9 million for amounts due to Neustar for estimated collections from Security Services customers of any billings after the closing date and until the customer contracts are assigned to Neustar. As a result of the sale, the Company recognized a pre-tax gain of approximately $54.8 million, which is included in Non-operating income in the fourth quarter of 2018. The estimated contingent consideration to be received, the liability for estimated future billings to be remitted to Neustar, and the gain recognized in 2018, are based on the Company’s best estimates of the probability and timing of customers consenting to the assignment of their contracts to Neustar. To the extent that the actual results differ from the Company’s estimates, the gain on the sale may be adjusted in 2019.