(3) Long‑term Debt

Long‑term debt consisted of the following as of December 31, 2015 and 2014 (in thousands):


	December 31,		December 31,
	2015		2014
Notes payable to banks, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 1.29% to 2.22% through 2016 to 2020, secured by aircraft	$	108,348	$	174,159
Notes payable to a financing company, due in semi-annual installments, variable interest based on LIBOR, or with interest rates ranging from 1.76% to 3.25% through 2017 to 2021, secured by aircraft		217,341		350,177
Notes payable to banks, due in semi-annual installments plus interest at 6.06% to 6.51% through 2021, secured by aircraft		108,069		129,201
Notes payable to a financing company, due in semi-annual installments plus interest at 5.78% to 6.23% through 2017, secured by aircraft		17,208		25,090
Notes payable to banks, due in monthly installments plus interest of 2.68% to 6.86% through 2025, secured by aircraft		479,170		572,446
Notes payable to banks, due in monthly installments, plus interest at 6.05% through 2020, secured by aircraft		11,304		13,551
Notes payable to banks, due in monthly installments, plus interest at 3.10% through 2019, secured by aircraft		4,615		5,909
Notes payable to banks, due in quarterly installments plus interest at 3.39% to 4.02% through 2027, secured by aircraft		966,156		446,724
Notes payable to banks, due in monthly installments, plus interest based on LIBOR at 3.21% to 3.33% through 2017, secured by aircraft		14,538		28,554
Notes payable to banks due in monthly installments, interest at 3.30% through 2019, secured by spare engines		22,054		—
Long-term debt	$	1,948,803	$	1,745,811
Less current maturities		(272,027)		(211,821)
Long-term debt, net of current maturities	$	1,676,776	$	1,533,990

During the year ended December 31, 2015, the Company acquired 25 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, 2015, the Company had $1.9 billion of long‑term debt obligations primarily related to the acquisition of CRJ200, CRJ700, CRJ900 and E175 aircraft. The average effective interest rate on the debt related to those long-term debt obligations was approximately 3.7% at December 31, 2015.

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, net of the write off of deferred loan costs associated with the debt, reflected as other income in the consolidated statements of comprehensive income (loss), of which the Company used $94 million in cash to pay off $128 million in debt, resulting in a pre-tax gain of $33 million during the fourth quarter of 2015.

The aggregate amounts of principal maturities of long‑term debt as of December 31, 2015 were as follows (in thousands):


2016	$	272,027
2017		248,629
2018		230,681
2019		223,898
2020		183,620
Thereafter		789,948
	$	1,948,803

As of December 31, 2015 and 2014, SkyWest Airlines had a $25 million line of credit. As of December 31, 2015 and 2014, SkyWest Airlines had no amount outstanding under the facility. However, at December 31, 2015 and 2014 the Company had $6 million and $5 million, respectively, in letters of credit issued under the facility which reduced the amount available under the facility to $19 million and $20 million, respectively. The facility expires on April 19, 2016 and has a variable interest rate of Libor plus 3.0%.

As of December 31, 2015 and 2014, the Company had $88.9 million and $79.9 million, respectively, in letters of credit and surety bonds outstanding with various banks and surety institutions.