Business Segments - Schedule of results of operation, depreciation and amortization, capital expenditures and total assets by reportable segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Jun. 30, 2018
Net Sales:
Consolidated net sales	$ 556.5	$ 487.8	$ 1,128.9	$ 967.5
Operating Income:
Consolidated operating income	55.4	41.4	100.4	83.6
Depreciation and Amortization:
Consolidated depreciation and amortization	30.3	29.2	60.1	58.0
Capital Expenditures:
Consolidated capital expenditures	40.1	26.8	81.7	55.7
Total Assets:
Consolidated total assets	3,161.8		3,161.8		$ 3,007.0
Corporate
Operating Income:
Consolidated operating income	(18.4)	(13.9)	(34.2)	(26.9)
Depreciation and Amortization:
Consolidated depreciation and amortization	1.2	0.9	2.4	1.8
Capital Expenditures:
Consolidated capital expenditures	6.5	9.6	14.2	16.7
Total Assets:
Consolidated total assets	173.6		173.6		193.2
Intersegment
Net Sales:
Consolidated net sales	(18.0)	(23.3)	(32.8)	(41.1)
Operating Income:
Consolidated operating income	0.4	(2.0)	1.2	(2.6)
Depreciation and Amortization:
Consolidated depreciation and amortization	(0.2)	(0.2)	(0.3)	(0.4)
Capital Expenditures:
Consolidated capital expenditures	(0.5)	(0.3)	(0.8)	(0.5)
Total Assets:
Consolidated total assets	(11.0)		(11.0)		(11.9)
Specialty Alloys Operations | Operating
Net Sales:
Consolidated net sales	461.6	406.3	937.1	803.1
Operating Income:
Consolidated operating income	69.0	49.8	121.7	100.3
Depreciation and Amortization:
Consolidated depreciation and amortization	23.8	23.4	47.5	46.5
Capital Expenditures:
Consolidated capital expenditures	20.8	11.2	41.4	28.4
Total Assets:
Consolidated total assets	2,341.9		2,341.9		2,312.1
Performance Engineered Products | Operating
Net Sales:
Consolidated net sales	112.9	104.8	224.6	205.5
Operating Income:
Consolidated operating income	4.4	7.5	11.7	12.8
Depreciation and Amortization:
Consolidated depreciation and amortization	5.5	5.1	10.5	10.1
Capital Expenditures:
Consolidated capital expenditures	13.3	$ 6.3	26.9	$ 11.1
Total Assets:
Consolidated total assets	$ 657.3		$ 657.3		$ 513.6