QUARTERLY RESULTS OF OPERATIONS (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Millions	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:
Telecommunications services	$ 423.9	$ 420.0	$ 433.8	$ 450.2	$ 454.8	$ 448.6	$ 326.7	$ 260.7	$ 1,727.8	$ 1,490.8	$ 827.9
Operating expenses:
Cost of telecommunications services	229.9	232.8	237.5	241.8	251.2	247.4	179.3	141.5	941.9	819.4	432.1
Operating income	23.5	30.1	31.1	38.7	11.8	14.5	4.2	9.2	123.3	39.7	25.4
Net loss	$ (19.1)	$ (26.2)	$ (33.3)	$ (27.3)	$ (53.0)	$ (23.4)	$ (136.3)	$ (30.7)	$ (105.9)	$ (243.4)	$ (71.5)
Loss per share:
Basic (in dollars per share)	$ (0.34)	$ (0.46)	$ (0.59)	$ (0.49)	$ (0.96)	$ (0.43)	$ (2.83)	$ (0.69)	$ (1.88)	$ (4.80)	$ (1.71)
Diluted (in dollars per share)	$ (0.34)	$ (0.46)	$ (0.59)	$ (0.49)	$ (0.96)	$ (0.43)	$ (2.83)	$ (0.69)	$ (1.88)	$ (4.80)	$ (1.71)
Weighted average shares:
Basic (in shares)	56,580,466	56,370,178	56,248,530	55,839,212	55,191,175	54,671,787	48,221,341	44,632,365	56,265,166	50,718,279	41,912,952
Diluted (in shares)	56,580,466	56,370,178	56,248,530	55,839,212	55,191,175	54,671,787	48,221,341	44,632,365	56,265,166	50,718,279	41,912,952