Total Debt - Schedule of Total Debt Outstanding (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Outstanding Principal	$ 1,142.0	$ 927.5
Premium	0.0	2.5
Issuance Costs and Fees	12.8	8.3
Long-Term Debt, Net	1,129.2	921.7
Current maturities of long-term debt	4.0	14.2
Total principal amount of debt, net of current maturities	1,138.0	913.3
Total debt, net of current maturities	1,125.2	907.5
2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	642.0
Premium	0.0
Issuance Costs and Fees	5.1
Long-Term Debt, Net	636.9
2014 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal		327.5
Premium		0.0
Issuance Costs and Fees		0.5
Long-Term Debt, Net		327.0
2028 Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Outstanding Principal	500.0
Premium	0.0
Issuance Costs and Fees	7.7
Long-Term Debt, Net	492.3
2021 Senior Notes \| Senior Notes
Debt Instrument [Line Items]
Outstanding Principal		600.0
Premium		2.5
Issuance Costs and Fees		7.8
Long-Term Debt, Net		594.7
Term Loan \| 2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	400.0
Premium	0.0
Issuance Costs and Fees	5.1
Long-Term Debt, Net	394.9
Term Loan \| 2014 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal		179.3
Premium		0.0
Issuance Costs and Fees		0.5
Long-Term Debt, Net		178.8
Revolving Credit Facility \| 2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	239.0
Premium	0.0
Issuance Costs and Fees	0.0
Long-Term Debt, Net	239.0
Swing line of credit \| 2017 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal	3.0
Premium	0.0
Issuance Costs and Fees	0.0
Long-Term Debt, Net	$ 3.0
Swing line of credit \| 2014 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal		13.2
Premium		0.0
Issuance Costs and Fees		0.0
Long-Term Debt, Net		13.2
Senior Secured Credit Facility due 2021 \| 2014 Credit Agreement \| Line of Credit
Debt Instrument [Line Items]
Outstanding Principal		135.0
Premium		0.0
Issuance Costs and Fees		0.0
Long-Term Debt, Net		$ 135.0

Long-term Debt, Excluding Current Maturities, Gross

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- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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-Publisher FASB
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-Section S99
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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- Definition

Amount, after accumulated amortization, of debt premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

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- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

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