(3) Long‑term Debt

Long‑term debt consisted of the following as of December 31, 2017 and 2016 (in thousands):


	December 31,		December 31,
	2017		2016
Notes payable to banks, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.91% to 3.43% through 2020, secured by aircraft	$	34,905	$	63,408
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 2.85% to 3.25% through 2021, secured by aircraft		97,612		146,266
Notes payable to banks, due in semi-annual installments plus interest at 6.06% to 6.51% through 2021, secured by aircraft		63,090		86,052
Notes payable to a financing company, due in semi-annual installments plus interest at 5.78% to 6.23% through 2017, secured by aircraft		—		8,854
Notes payable to banks, due in monthly installments plus interest of 2.68% to 6.86% through 2025, secured by aircraft		372,157		426,389
Notes payable to banks, due in monthly installments, plus interest at 4.16% to 6.05% through 2029, secured by aircraft		49,001		8,998
Notes payable to banks, due in monthly installments, plus interest at 3.44% through 2019, secured by aircraft		—		3,280
Notes payable to banks, due in quarterly installments, plus interest at 3.39% to 4.73% through 2029, secured by aircraft		2,085,822		1,806,981
Notes payable to banks, due in monthly installments, plus interest based on LIBOR at 3.54% through 2017, secured by aircraft		—		4,666
Notes payable to banks due in monthly installments, interest at 3.30% through 2019, secured by spare engines		9,763		16,010
Long-term debt	$	2,712,350	$	2,570,904
Current portion of long-term debt		(313,243)		(308,945)
Less long-term portion of unamortized debt issue cost, net		(21,761)		(21,908)
Long-term debt, net of current maturities and debt issue costs	$	2,377,346	$	2,240,051

Current portion of long-term debt		313,243		308,945
Less current portion of unamortized debt issue cost, net		(3,565)		(3,485)
Current portion of long-term debt, net of debt issue costs	$	309,678	$	305,460

During the year ended December 31, 2017, the Company acquired 21 new E175 aircraft. Approximately 85% of the aircraft purchase price was financed through the issuance of debt and 15% of the aircraft purchase price was paid with cash.

As of December 31, 2017 and 2016, the Company had $2.7 billion and $2.6 billion, respectively, of long‑term debt obligations primarily related to the acquisition of aircraft and certain spare engines. The average effective interest rate on the debt related to those long-term debt obligations at December 31, 2017 and 2016, was approximately 3.9% and 3.8%, respectively.

During the year ended December 31, 2016, the Company used $16.5 million in cash to extinguish $18.4 million in debt early. The payment resulted in a pre-tax gain of $1.3 million, reflected as other income in the consolidated statements of comprehensive income (loss). During the year ended December 31, 2015, the Company used $110.8 million in cash to pay off $145.4 million in debt. The payment resulted in a pre-tax gain of $33.7 million, reflected as other income in the consolidated statements of comprehensive income (loss). The Company did not extinguish any debt early during 2017.

The aggregate amounts of principal maturities of long‑term debt as of December 31, 2017 were as follows (in thousands):


2018	$	313,243
2019		315,753
2020		284,998
2021		272,470
2022		280,360
Thereafter		1,245,526
	$	2,712,350

As of December 31, 2017 and 2016, SkyWest Airlines had a $75 million line of credit. The line of credit includes minimum liquidity and profitability covenants and is secured by certain assets. As of December 31, 2017 and 2016, SkyWest Airlines had no amount outstanding under the facility. However, at December 31, 2017 and 2016 the Company had $14.8 million and $6.5 million, respectively, in letters of credit issued under the facility which reduced the amount available under the facility to $60.2 million and $68.5 million, respectively. The facility expires on June 1, 2018 and has a variable interest rate of LIBOR plus 2.5% (4.1% at December 31, 2017).

As of December 31, 2017 and 2016, the Company had $87.4 million and $87.7 million, respectively, in letters of credit and surety bonds outstanding with various banks and surety institutions.