STATEMENTS OF CONSOLIDATED CASH FLOWS - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net income		$ 28.8	$ 62.0	$ 91.7
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation of property, plant and equipment		49.3	46.3	42.1
Amortization of definite-lived intangible assets		2.9	2.8	1.8
Amortization of debt premium and debt issuance costs		2.0	1.2	1.0
Deferred income taxes		15.2	21.1	36.7
Non-cash equity compensation		10.0	9.8	10.3
Non-cash asset impairment charges		0.5	26.1	1.4
Loss on extinguishment of debt			20.3
Loss (gain) on disposition of property, plant and equipment		0.2		(0.2)
Gain on disposition of short-term investments		(0.7)	(0.9)	(2.9)
Non-cash defined benefit net periodic postretirement benefit cost		5.2	7.0	6.6
Other non-cash changes in assets and liabilities		(1.8)	8.6	21.9
Changes in operating assets and liabilities:
Trade and other receivables		60.8	20.2	(22.3)
Contract assets		18.5	0.3	0.7
Inventories		25.6	37.5	(45.0)
Prepaid expenses and other current assets		(3.2)	(1.7)	(0.4)
Accounts payable		(5.9)	(24.5)	29.2
Accrued liabilities		(3.9)	(7.5)	(2.6)
Annual variable cash contributions to VEBAs		(2.9)	(2.1)	(15.7)
Long-term assets and liabilities, net		6.3	5.8	(4.1)
Net cash provided by operating activities		206.9	232.3	150.2
Cash flows from investing activities:
Capital expenditures		(51.9)	(60.2)	(74.1)
Purchase of short-term investments		(39.1)	(132.2)	(135.2)
Purchase of equity securities		(0.5)	(0.7)	(0.9)
Proceeds from disposition of short-term investments		118.1	91.1	283.9
Cash payment for acquisition of Imperial Machine & Tool Co., net of cash received				(43.2)
Proceeds from disposal of property, plant and equipment		0.3	0.2	0.6
Net cash provided by (used in) investing activities	[1]	26.9	(101.8)	31.1
Cash flows from financing activities:
Repayment of principal and redemption premium of 5.875% Senior Notes			(391.5)
Issuance of 6.50% and 4.625% Senior Notes		350.0	500.0
Cash paid for debt issuance costs		(6.2)	(8.8)
Repayment of finance lease		(1.7)	(1.4)	(0.7)
Cancellation of shares to cover employees' tax withholdings upon vesting of non-vested shares		(4.3)	(6.2)	(6.9)
Repurchase of common stock		(12.5)	(44.2)	(60.7)
Cash dividends and dividend equivalents paid		(43.4)	(39.4)	(37.7)
Net cash provided by (used in) financing activities	[1]	281.9	8.5	(106.0)
Net increase in cash, cash equivalents and restricted cash during the period		515.7	139.0	75.3
Cash, cash equivalents and restricted cash at beginning of period		278.6	139.6	64.3
Cash, cash equivalents and restricted cash at end of period		$ 794.3	$ 278.6	$ 139.6

[1] See Note 16 for supplemental disclosure of cash flow information.