Long-term Debt (Details) $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($) Rate	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Unamortized deferred loan costs \| $	$ 24,846	$ 12,787
Long-term Debt \| $	1,154,154	$ 575,213
Line of Credit Facility, Maximum Borrowing Capacity \| $	$ 300,000
Pro Forma Consolidated Leverage Ratio	3.25%
FeesPaidInConnectionWithAmendment \| $	$ 22,450
Unamortized Loan Commitment and Origination Fees and Unamortized Discounts or Premiums \| $	15,300
Interest Expense, Debt \| $	6,160
Contractual Quarterly Payments \| $	$ 2,100
Leverage Ratio	2.89
$822 Million Term B Loan Facility, due April 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument, Face Amount \| $	$ 822,000
Maximum [Member]
Debt Instrument [Line Items]
Leverage Ratio	4.75%
Leverage Ratio in Latest Year of Debt Term	3.75%
Notes Payable to Banks [Member] \| Minimum [Member] \| $200 Million Revolving Credit Facility, Due March 2023 [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 March 2023 [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 March 2023 [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] \| $200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Basis Spread on Variable Rate	2.25%
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%
$200 Million Revolving Credit Facility, Due March 2023 [Member]
Debt Instrument [Line Items]
Line of Credit Facility, Remaining Borrowing Capacity \| $	$ 195,600

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Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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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.

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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.

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