Supplemental Guarantor Information HY Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities		$ 173.2	$ 110.9	$ 175.2
Payments to Acquire Property, Plant, and Equipment		(286.4)	(283.6)	(182.3)
Proceeds from Dividends Received		2.1	22.2	28.4
Proceeds from Sale of Available-for-sale Securities, Equity	$ 100.0	189.7	643.9	355.9
Payments for (Proceeds from) Other Investing Activities		(0.9)	0.7	(3.8)
Net Cash Provided by (Used in) Investing Activities		(95.5)	383.2	392.6
Proceeds from issuance of long-term debt		635.0	0.0	0.0
Net (decrease) increase in corporate credit and receivables facilities with initial maturities less than 90 days		71.9	(1.6)	(127.0)
Repayment of debt		(759.3)	(531.7)	(376.5)
Debt issuance costs		(11.1)	(0.4)	(0.9)
Payments for Repurchase of Common Stock		(4.8)	0.0	0.0
Proceeds from (Payments for) Other Financing Activities		3.3	(0.5)	0.3
Net Cash Provided by (Used in) Financing Activities		(75.4)	(544.6)	(514.5)
Cash and Cash Equivalents, Period Increase (Decrease)		2.3	(50.5)	53.3
Cash and cash equivalents		9.7	7.4	57.9	$ 4.6
Notes guaranteed by subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities		173.2	110.9	175.2
Payments to Acquire Property, Plant, and Equipment		(286.4)	(283.6)	(182.3)
Proceeds from Dividends Received		2.1	22.2	28.4
Proceeds from Sale of Available-for-sale Securities, Equity		189.7	643.9	355.9
Proceeds from sale of assets				194.4
Distributions Received from Subsidiaries		0.0	0.0	0.0
Funding Between Parent and Subsidiaries, net investing		0.0	0.0	0.0
Payments for (Proceeds from) Other Investing Activities		(0.9)	0.7	(3.8)
Net Cash Provided by (Used in) Investing Activities		(95.5)	383.2	392.6
Funding between Parent and subsidiaries, net financing		0.0	0.0	0.0
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries		0.0	0.0	0.0
Proceeds from issuance of long-term debt		635.0
Net (decrease) increase in corporate credit and receivables facilities with initial maturities less than 90 days		71.9	(1.6)	(127.0)
Repayment of debt		(759.3)	(531.7)	(376.5)
Debt issuance costs		(11.1)	(0.4)	(0.9)
Proceeds from (Payments for) Other Financing Activities		(11.9)	(10.9)	(10.1)
Net Cash Provided by (Used in) Financing Activities		(75.4)	(544.6)	(514.5)
Cash and Cash Equivalents, Period Increase (Decrease)		2.3	(50.5)	53.3
Cash and cash equivalents		9.7	7.4	57.9	4.6
Notes guaranteed by subsidiaries [Member] | Consolidation, Eliminations [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities		0.0	0.0	0.0
Payments to Acquire Property, Plant, and Equipment		0.0	0.0	0.0
Proceeds from Dividends Received		0.0	0.0	0.0
Proceeds from Sale of Available-for-sale Securities, Equity		0.0	0.0	0.0
Proceeds from sale of assets				0.0
Distributions Received from Subsidiaries		(12.0)	(11.3)	(12.8)
Funding Between Parent and Subsidiaries, net investing		36.0	482.4	617.3
Payments for (Proceeds from) Other Investing Activities		0.0	0.0	0.0
Net Cash Provided by (Used in) Investing Activities		24.0	471.1	604.5
Funding between Parent and subsidiaries, net financing		(36.0)	(482.4)	(617.3)
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries		12.0	11.3	12.8
Proceeds from issuance of long-term debt		0.0
Net (decrease) increase in corporate credit and receivables facilities with initial maturities less than 90 days		0.0	0.0	0.0
Repayment of debt		0.0	0.0	0.0
Debt issuance costs		0.0	0.0	0.0
Proceeds from (Payments for) Other Financing Activities		0.0	0.0	0.0
Net Cash Provided by (Used in) Financing Activities		(24.0)	(471.1)	(604.5)
Cash and Cash Equivalents, Period Increase (Decrease)		0.0	0.0	0.0
Cash and cash equivalents		0.0	0.0	0.0	0.0
Notes guaranteed by subsidiaries [Member] | Parent Company [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities		(61.1)	(19.3)	(56.3)
Payments to Acquire Property, Plant, and Equipment		(0.2)	(0.1)	(0.2)
Proceeds from Dividends Received		0.0	0.0	0.0
Proceeds from Sale of Available-for-sale Securities, Equity		0.0	0.0	0.0
Proceeds from sale of assets				0.0
Distributions Received from Subsidiaries		12.0	11.3	12.8
Funding Between Parent and Subsidiaries, net investing		152.0	0.0	0.0
Payments for (Proceeds from) Other Investing Activities		(0.9)	(0.3)	(0.3)
Net Cash Provided by (Used in) Investing Activities		162.9	10.9	12.3
Funding between Parent and subsidiaries, net financing		0.0	486.4	516.2
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries		0.0	0.0	0.0
Proceeds from issuance of long-term debt		635.0
Net (decrease) increase in corporate credit and receivables facilities with initial maturities less than 90 days		0.0	0.0	(40.0)
Repayment of debt		(710.9)	(518.5)	(367.3)
Debt issuance costs		(10.8)	(0.2)	(0.7)
Proceeds from (Payments for) Other Financing Activities		(11.9)	(10.9)	(10.1)
Net Cash Provided by (Used in) Financing Activities		(98.6)	(43.2)	98.1
Cash and Cash Equivalents, Period Increase (Decrease)		3.2	(51.6)	54.1
Cash and cash equivalents		7.8	4.6	56.2	2.1
Notes guaranteed by subsidiaries [Member] | Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities		25.0	(86.8)	5.6
Payments to Acquire Property, Plant, and Equipment		(25.4)	(22.5)	(29.6)
Proceeds from Dividends Received		2.1	22.2	28.4
Proceeds from Sale of Available-for-sale Securities, Equity		189.7	643.9	355.9
Proceeds from sale of assets				194.4
Distributions Received from Subsidiaries		0.0	0.0	0.0
Funding Between Parent and Subsidiaries, net investing		(188.0)	(554.3)	(546.3)
Payments for (Proceeds from) Other Investing Activities		0.0	0.9	(5.5)
Net Cash Provided by (Used in) Investing Activities		(21.6)	90.2	(2.7)
Funding between Parent and subsidiaries, net financing		0.0	0.0	0.0
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries		0.0	0.0	0.0
Proceeds from issuance of long-term debt		0.0
Net (decrease) increase in corporate credit and receivables facilities with initial maturities less than 90 days		0.0	0.0	0.0
Repayment of debt		(3.5)	(3.2)	(3.0)
Debt issuance costs		0.0	0.0	0.0
Proceeds from (Payments for) Other Financing Activities		0.0	0.0	0.0
Net Cash Provided by (Used in) Financing Activities		(3.5)	(3.2)	(3.0)
Cash and Cash Equivalents, Period Increase (Decrease)		(0.1)	0.2	(0.1)
Cash and cash equivalents		0.3	0.4	0.2	0.3
Notes guaranteed by subsidiaries [Member] | Non-Guarantor Subsidiaries [Member]
Condensed Financial Statements, Captions [Line Items]
Net Cash Provided by (Used in) Operating Activities		209.3	217.0	225.9
Payments to Acquire Property, Plant, and Equipment		(260.8)	(261.0)	(152.5)
Proceeds from Dividends Received		0.0	0.0	0.0
Proceeds from Sale of Available-for-sale Securities, Equity		0.0	0.0	0.0
Proceeds from sale of assets				0.0
Distributions Received from Subsidiaries		0.0	0.0	0.0
Funding Between Parent and Subsidiaries, net investing		0.0	71.9	(71.0)
Payments for (Proceeds from) Other Investing Activities		0.0	0.1	2.0
Net Cash Provided by (Used in) Investing Activities		(260.8)	(189.0)	(221.5)
Funding between Parent and subsidiaries, net financing		36.0	(4.0)	101.1
Cash Dividends Paid to Parent Company by Consolidated Subsidiaries		(12.0)	(11.3)	(12.8)
Proceeds from issuance of long-term debt		0.0
Net (decrease) increase in corporate credit and receivables facilities with initial maturities less than 90 days		71.9	(1.6)	(87.0)
Repayment of debt		(44.9)	(10.0)	(6.2)
Debt issuance costs		(0.3)	(0.2)	(0.2)
Proceeds from (Payments for) Other Financing Activities		0.0	0.0	0.0
Net Cash Provided by (Used in) Financing Activities		50.7	(27.1)	(5.1)
Cash and Cash Equivalents, Period Increase (Decrease)		(0.8)	0.9	(0.7)
Cash and cash equivalents		$ 1.6	$ 2.4	$ 1.5	$ 2.2