RESTRUCTURING, IMPAIRMENT AND OTHER CHARGES: Narrative (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		9 Months Ended	12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2018	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments	$ 5,043	$ (765)	$ 5,534	$ 2,066
Restructuring reserve					$ 6,000
Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments					5,500	$ 2,800	$ 6,500
Restructuring Activity2015 | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments								$ 9,200
Employee Severance | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments						200
Employee Severance | Restructuring Activity 2018
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments					3,900
Restructuring reserve					2,800
Employee Severance | Restructuring Activity2015 | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments								2,600
Termination Of Associates | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments					800	$ 2,600	3,000
Leased office facilities | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments							1,500
Adjustments to estimates related to the fiscal 2015 lease accruals | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments							1,500
Contract Termination | Restructuring Activity2015 | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments								4,700
Leasehold Improvement Write Offs | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments					$ 800		$ 2,100
Leasehold Improvement Write Offs | Restructuring Activity2015 | Gains Losses And Other Items Net
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments								2,000
United States Australia China And Europe | Employee Severance | Restructuring Activity2015
Restructuring Cost and Reserve [Line Items]
Restructuring charges and adjustments								$ 2,600
Restructuring reserve	$ 300		$ 300