(5) Long-Term Debt

Long-term debt at December 31, 2016 and 2015 consisted of the following (in thousands):

	2016			2015
Long-term debt, including current portion:
$550,000,000 revolving credit facility due April 30, 2020	$	225,986		$	278,834
$150,000,000 senior notes Series A due February 27, 2020		150,000			150,000
$350,000,000 senior notes Series B due February 27, 2023		350,000			350,000
$10,000,000 credit line due June 30, 2017		—			—
	$	725,986		$	778,834
Unamortized debt issuance costs		(3,184	)		(3,985	)
	$	722,802		$	774,849

The aggregate payments due on the long-term debt in each of the next five years were as follows (in thousands):

2017	$	—
2018		—
2019		—
2020		375,986
2021		—
Thereafter		350,000
	$	725,986

The Company has a $550,000,000 unsecured revolving credit facility (“Revolving Credit Facility”) with a syndicate of banks, with JPMorgan Chase Bank, N.A. as the administrative agent bank, with a maturity date of April 30, 2020. In addition, the credit agreement allows for a $300,000,000 increase in the aggregate commitments of the banks in the form of revolving credit loans or term loans, subject to the consent of each bank that elects to participate in the increased commitment. The variable interest rate spread varies with the Company’s senior debt rating and is currently 1.00% over the London Interbank Offered Rate (“LIBOR”) or equal to an alternate base rate calculated with reference to the agent bank’s prime rate, among other factors (“Alternate Base Rate”). The commitment fee is currently 0.10%. The Revolving Credit Facility contains certain restrictive financial covenants including an interest coverage ratio and a debt-to-capitalization ratio. In addition to financial covenants, the Revolving Credit Facility contains covenants that, subject to exceptions, restrict debt incurrence, mergers and acquisitions, sales of assets, dividends and investments, liquidations and dissolutions, capital leases, transactions with affiliates and changes in lines of business. Borrowings under the Revolving Credit Facility may be used for general corporate purposes, the purchase of existing or new equipment, the purchase of the Company’s common stock, or for business acquisitions. As of December 31, 2016, the Company was in compliance with all Revolving Credit Facility covenants and had $225,986,000 of debt outstanding under the Revolving Credit Facility. The average borrowing under the Revolving Credit Facility during 2016 was $249,247,000, computing by averaging the daily balance, and the weighted average interest rate was 1.5%, computed by dividing the interest expense under the Revolving Credit Facility by the average Revolving Credit Facility borrowing. The Revolving Credit Facility includes a $25,000,000 commitment which may be used for standby letters of credit. Outstanding letters of credit under the Revolving Credit Facility were $2,518,000 as of December 31, 2016.

The Company has $500,000,000 of unsecured senior notes (“Senior Notes Series A” and “Senior Notes Series B”) with a group of institutional investors, consisting of $150,000,000 of 2.72% Senior Notes Series A due February 27, 2020 and $350,000,000 of 3.29% Senior Notes Series B due February 27, 2023. No principal payments are required until maturity. The Senior Notes Series A and Series B contain certain covenants on the part of the Company, including an interest coverage covenant, a debt-to-capitalization covenant and covenants relating to liens, asset sales and mergers, among others. The Senior Notes Series A and Series B also specify certain events of default, upon the occurrence of which the maturity of the notes may be accelerated, including failure to pay principal and interest, violation of covenants or default on other indebtedness, among others. As of December 31, 2016, the Company was in compliance with all Senior Notes Series A and Series B covenants and had $150,000,000 of Senior Notes Series A outstanding and $350,000,000 of Senior Notes Series B outstanding.

The Company has a $10,000,000 line of credit (“Credit Line”) with Bank of America, N.A. (“Bank of America”) for short-term liquidity needs and letters of credit, with a maturity date of June 30, 2017. The Credit Line allows the Company to borrow at an interest rate agreed to by Bank of America and the Company at the time each borrowing is made or continued. The Company had no borrowings outstanding under the Credit Line as December 31, 2016. Outstanding letters of credit under the Credit Line were $1,012,000 as of December 31, 2016.

The estimated fair value of total debt outstanding at December 31, 2016 and 2015 was $715,330,000 and $764,781,000, respectively, which differs from the carrying amount of $722,802,000 and $774,849,000, respectively, included in the consolidated financial statements. The fair value was determined using an income approach that relies on inputs such as yield curves.