Borrowings (Schedule of Borrowings) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Principal components:
Total principal outstanding	$ 1,105,000	$ 1,200,000
Premium on senior unsecured notes, net of accumulated amortization	5,906	6,469
Unamortized deferred financing costs	(15,646)	(16,996)
Total carrying value of borrowings (a)	1,095,260	1,189,473
Deferred financing costs	5,427	6,087
Senior secured revolving credit facility
Principal components:
Total principal outstanding	105,000	200,000
6.125% senior unsecured notes due 2024
Principal components:
Total principal outstanding	$ 600,000	600,000
Debt instrument, interest rate, stated (as a percent)	6.125%
6.375% senior unsecured notes due 2026
Principal components:
Total principal outstanding	$ 400,000	$ 400,000
Debt instrument, interest rate, stated (as a percent)	6.375%

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.

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Amount, after accumulated amortization, of debt discount (premium).

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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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