10. Long-Term Debt

Debt is summarized below (in thousands):

	June 30, 2013		December 31, 2012
Credit Facility
Credit Facility, variable interest, due September 2017 (1)	$	—	$	—

Senior Notes (2)
2018 Senior Notes, 8.75% interest, net of discount of zero and $109, respectively, issued April and May 2008	—		81,003
2020 Senior Notes, 6.75% interest, issued November 2010 and due November 2020	500,000		500,000
2021 Senior Notes, 6.5% interest, net of discount of $505 and $826, respectively, issued February and March 2011 and due August 2021	324,495		499,174
2022 Senior Notes, 6.25% interest, issued October 2011 and due June 2022	455,000		700,000
2023A Senior Notes, 5.5% interest, net of discount of $6,791 and $7,126, respectively, issued August 2012 and due February 2023	743,209		742,874
2023B Senior Notes, 4.5% interest, issued January 2013 and due July 2023	1,000,000		—
Total long-term debt	$	3,022,704	$	2,523,051

(1) Applicable interest rate was 5.00% at June 30, 2013.

(2) The estimated aggregate fair value of the senior notes (collectively, the “Senior Notes”) was approximately $2,989.9 million and $2,763 million as of June 30, 2013 and December 31, 2012, respectively, based on quoted prices in an inactive market. The fair value of the Partnership’s Senior Notes is considered a Level 3 measurement.

Credit Facility

Under the provisions of the Credit Facility, the Partnership is subject to a number of restrictions and covenants. These covenants are used to calculate the available borrowing capacity on a quarterly basis. The Credit Facility is guaranteed by the Partnership’s 100% owned subsidiaries, other than MarkWest Liberty Midstream and its subsidiaries, and collateralized by substantially all of the Partnership’s assets and those of its 100% owned subsidiaries, other than MarkWest Liberty Midstream and its subsidiaries. As of June 30, 2013, the Partnership had no borrowings outstanding and approximately $11.3 million of letters of credit outstanding under the Credit Facility, leaving approximately $1,188.7 million of unused capacity of which approximately $271.5 million was available for borrowing based on financial covenant requirements. Additionally, the full amount of unused capacity is available for borrowing on a short term basis to provide financial flexibility within a given fiscal quarter.

Senior Notes

In January 2013, the Partnership completed a public offering for $1 billion in aggregate principal amount of 4.5% senior unsecured notes due July 2023. The Partnership received net proceeds of approximately $986.0 million after deducting underwriters’ and third-party expenses. A portion of the proceeds, together with cash on hand, was used to repurchase $81.1 million aggregate principal amount of the Partnership’s 8.75% senior notes due April 2018, $175.0 million of the outstanding principal amount of the Partnership’s 6.5% senior notes due August 2021 and $245 million of the outstanding principal amount of the Partnership’s 6.25% senior notes due June 2022, with the remainder used to fund the Partnership’s capital expenditure program and for general partnership purposes. The Partnership recorded a total pre-tax loss of approximately $38.5 million related to the repurchases. The pre-tax loss consisted of approximately $7.0 million related to the non-cash write-off of the unamortized discount and deferred finance costs and approximately $31.5 million related to the payment of redemption premiums.