Segment Information (Revenue by Customer Type) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018		Dec. 31, 2017
Revenue, Major Customer [Line Items]
Revenues	$ 809,823	$ 732,901	$ 748,697	$ 719,143	$ 801,870	$ 732,476	$ 742,365	$ 697,192	$ 3,010,564	$ 2,973,903	[1],[2]	$ 2,824,686	[1],[2]
Strategic Service Provider
Revenue, Major Customer [Line Items]
Revenues									119,929	152,995		174,881	[3]
Non-strategic Service Provider
Revenue, Major Customer [Line Items]
Revenues									$ 2,890,635	$ 2,820,908		$ 2,649,805	[3]

[1] As noted above, prior period amounts have not been adjusted under the modified retrospective method of adoption of the revenue recognition standard.

[2] Prior period amounts have not been adjusted under the modified retrospective method of adoption of the revenue recognition standard.

[3] As noted above, prior period amounts have not been adjusted under the modified retrospective method of adoption of the revenue recognition standard.