Long-term Debt (Details) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($) Rate	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Unamortized deferred loan costs \| $	$ 12,071	$ 12,787
Long-term Debt \| $	565,929	$ 575,213
Contractual Quarterly Payments \| $	$ 2,100
Leverage Ratio	1.80
$825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount \| $	$ 825,000
Maximum [Member]
Debt Instrument [Line Items]
Leverage Ratio in Latest Year of Debt Term	3.25
Leverage Ratio	3.75
Notes Payable to Banks [Member] \| Minimum [Member] \| $200 Million Revolving Credit Facility, Due February 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	0.25%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.20%
Notes Payable to Banks [Member] \| Minimum [Member] \| Eurodollar [Member] \| $200 Million Revolving Credit Facility, Due February 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Notes Payable to Banks [Member] \| Maximum [Member] \| $200 Million Revolving Credit Facility, Due February 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	1.25%
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage	0.35%
Notes Payable to Banks [Member] \| Maximum [Member] \| Eurodollar [Member] \| $825 Million Term B Loan Facility, due June 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.00%
Notes Payable to Banks [Member] \| Maximum [Member] \| Eurodollar [Member] \| $200 Million Revolving Credit Facility, Due February 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
$200 Million Revolving Credit Facility, Due February 2022 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity \| $	$ 195,700

Contractual Quarterly Payments

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- Definition

The leverage ratio as defined in the credit agreement.

+ References

No definition available.

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- Definition

Leverage ratio limit in latest year of debt facility, as described in debt agreement.

+ References

No definition available.

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- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

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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 of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19(b),22(b))
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- Definition

The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility.

+ References

No definition available.

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- Definition

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
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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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-Name Accounting Standards Codification
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