Guarantor Subsidiaries (Consolidating Statement of Cash Flow) (Details) - USD ($) $ in Millions	3 Months Ended				6 Months Ended		12 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Operating Activities
Net income	$ 168.9	$ 89.3	$ 32.7	$ 5.3	$ 258.2	$ 38.0
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	39.5		36.5		78.9	76.2
Deferred and noncurrent income tax charges (credits)					9.4	2.7
Accretion of asset retirement obligations	0.5		0.5		1.1	1.0	$ 2.1
(Gain) loss on sale of assets	(1.3)		0.0		(1.4)	0.1
Net (increase) decrease in noncash operating working capital					23.2	(0.5)
Equity in earnings of affiliates	0.0		0.0		0.0	0.0
Other operating activities - net					12.5	7.4
Net cash provided by (required by) operating activities					381.9	124.9
Investing Activities
Property additions					(105.7)	(86.6)
Proceeds from sale of assets					7.6	1.4
Other investing activities - net					(1.1)	(0.5)
Net cash provided by (required by) investing activities					(99.2)	(85.7)
Financing Activities
Purchase of treasury stock					(140.6)	(30.1)
Borrowings of debt					0.0
Repayments of debt					(13.2)	(10.7)
Amounts related to share-based compensation					(5.6)	(4.3)
Net distributions to parent					0.0	0.0
Net cash provided by (required by) financing activities					(159.4)	(45.1)
Net increase (decrease) in cash, cash equivalents, and restricted cash					123.3	(5.9)
Cash, cash equivalents, and restricted cash at beginning of period		280.3		184.5	280.3	184.5	184.5
Cash, cash equivalents, and restricted cash at end of period	403.6		178.6		403.6	178.6	280.3
Eliminations
Operating Activities
Net income	(168.7)		(32.6)		(257.9)	(37.7)
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	0.0		0.0		0.0	0.0
Deferred and noncurrent income tax charges (credits)					0.0	0.0
Accretion of asset retirement obligations	0.0		0.0		0.0	0.0
(Gain) loss on sale of assets	0.0		0.0		0.0	0.0
Net (increase) decrease in noncash operating working capital					0.0	0.0
Equity in earnings of affiliates	168.7		32.6		257.9	37.7
Other operating activities - net					0.0	0.0
Net cash provided by (required by) operating activities					0.0	0.0
Investing Activities
Property additions					0.0	0.0
Proceeds from sale of assets					0.0	0.0
Other investing activities - net					0.0	0.0
Net cash provided by (required by) investing activities					0.0	0.0
Financing Activities
Purchase of treasury stock					0.0	0.0
Borrowings of debt					0.0
Repayments of debt					0.0	0.0
Amounts related to share-based compensation					0.0	0.0
Net distributions to parent					0.0	0.0
Net cash provided by (required by) financing activities					0.0	0.0
Net increase (decrease) in cash, cash equivalents, and restricted cash					0.0	0.0
Cash, cash equivalents, and restricted cash at beginning of period		0.0		0.0	0.0	0.0	0.0
Cash, cash equivalents, and restricted cash at end of period	0.0		0.0		0.0	0.0	0.0
Parent Company | Reportable Legal Entities
Operating Activities
Net income	168.9		32.7		258.2	38.0
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	0.0		0.0		0.0	0.0
Deferred and noncurrent income tax charges (credits)					0.0	0.0
Accretion of asset retirement obligations	0.0		0.0		0.0	0.0
(Gain) loss on sale of assets	0.0		0.0		0.0	0.0
Net (increase) decrease in noncash operating working capital					0.0	0.0
Equity in earnings of affiliates	(168.9)		(32.7)		(258.2)	(38.0)
Other operating activities - net					0.0	0.0
Net cash provided by (required by) operating activities					0.0	0.0
Investing Activities
Property additions					0.0	0.0
Proceeds from sale of assets					0.0	0.0
Other investing activities - net					0.0	0.0
Net cash provided by (required by) investing activities					0.0	0.0
Financing Activities
Purchase of treasury stock					(140.6)	(30.1)
Borrowings of debt					0.0
Repayments of debt					0.0	0.0
Amounts related to share-based compensation					0.0	0.0
Net distributions to parent					140.6	30.1
Net cash provided by (required by) financing activities					0.0	0.0
Net increase (decrease) in cash, cash equivalents, and restricted cash					0.0	0.0
Cash, cash equivalents, and restricted cash at beginning of period		0.0		0.0	0.0	0.0	0.0
Cash, cash equivalents, and restricted cash at end of period	0.0		0.0		0.0	0.0	0.0
Issuer | Reportable Legal Entities
Operating Activities
Net income	168.9		32.7		258.2	38.0
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	39.4		36.5		78.8	76.2
Deferred and noncurrent income tax charges (credits)					9.4	2.7
Accretion of asset retirement obligations	0.5		0.5		1.1	1.0
(Gain) loss on sale of assets	(1.3)		0.0		(1.4)	0.1
Net (increase) decrease in noncash operating working capital					23.0	(0.4)
Equity in earnings of affiliates	0.2		0.1		0.3	0.3
Other operating activities - net					12.5	7.4
Net cash provided by (required by) operating activities					381.9	125.3
Investing Activities
Property additions					(102.8)	(83.4)
Proceeds from sale of assets					7.6	1.4
Other investing activities - net					(1.1)	(0.5)
Net cash provided by (required by) investing activities					(96.3)	(82.5)
Financing Activities
Purchase of treasury stock					0.0	0.0
Borrowings of debt					0.0
Repayments of debt					(13.2)	(10.7)
Amounts related to share-based compensation					(5.6)	(4.3)
Net distributions to parent					(143.2)	(33.8)
Net cash provided by (required by) financing activities					(162.0)	(48.8)
Net increase (decrease) in cash, cash equivalents, and restricted cash					123.6	(6.0)
Cash, cash equivalents, and restricted cash at beginning of period		279.4		184.0	279.4	184.0	184.0
Cash, cash equivalents, and restricted cash at end of period	403.0		178.0		403.0	178.0	279.4
Guarantor Subsidiaries | Reportable Legal Entities
Operating Activities
Net income	(0.2)		(0.1)		(0.3)	(0.3)
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	0.1		0.0		0.1	0.0
Deferred and noncurrent income tax charges (credits)					0.0	0.0
Accretion of asset retirement obligations	0.0		0.0		0.0	0.0
(Gain) loss on sale of assets	0.0		0.0		0.0	0.0
Net (increase) decrease in noncash operating working capital					0.2	(0.1)
Equity in earnings of affiliates	0.0		0.0		0.0	0.0
Other operating activities - net					0.0	0.0
Net cash provided by (required by) operating activities					0.0	(0.4)
Investing Activities
Property additions					(2.9)	(3.2)
Proceeds from sale of assets					0.0	0.0
Other investing activities - net					0.0	0.0
Net cash provided by (required by) investing activities					(2.9)	(3.2)
Financing Activities
Purchase of treasury stock					0.0	0.0
Borrowings of debt					0.0
Repayments of debt					0.0	0.0
Amounts related to share-based compensation					0.0	0.0
Net distributions to parent					2.6	3.7
Net cash provided by (required by) financing activities					2.6	3.7
Net increase (decrease) in cash, cash equivalents, and restricted cash					(0.3)	0.1
Cash, cash equivalents, and restricted cash at beginning of period		0.9		0.5	0.9	0.5	0.5
Cash, cash equivalents, and restricted cash at end of period	0.6		0.6		0.6	0.6	0.9
Non-Guarantor Subsidiaries | Reportable Legal Entities
Operating Activities
Net income	0.0		0.0		0.0	0.0
Adjustments to reconcile net income (loss) to net cash provided by (required by) operating activities
Depreciation and amortization	0.0		0.0		0.0	0.0
Deferred and noncurrent income tax charges (credits)					0.0	0.0
Accretion of asset retirement obligations	0.0		0.0		0.0	0.0
(Gain) loss on sale of assets	0.0		0.0		0.0	0.0
Net (increase) decrease in noncash operating working capital					0.0	0.0
Equity in earnings of affiliates	0.0		0.0		0.0	0.0
Other operating activities - net					0.0	0.0
Net cash provided by (required by) operating activities					0.0	0.0
Investing Activities
Property additions					0.0	0.0
Proceeds from sale of assets					0.0	0.0
Other investing activities - net					0.0	0.0
Net cash provided by (required by) investing activities					0.0	0.0
Financing Activities
Purchase of treasury stock					0.0	0.0
Borrowings of debt					0.0
Repayments of debt					0.0	0.0
Amounts related to share-based compensation					0.0	0.0
Net distributions to parent					0.0	0.0
Net cash provided by (required by) financing activities					0.0	0.0
Net increase (decrease) in cash, cash equivalents, and restricted cash					0.0	0.0
Cash, cash equivalents, and restricted cash at beginning of period		$ 0.0		$ 0.0	0.0	0.0	0.0
Cash, cash equivalents, and restricted cash at end of period	$ 0.0		$ 0.0		$ 0.0	$ 0.0	$ 0.0