CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income (loss)		$ 280,372	$ 428,907		$ (161,586)
Adjustments to reconcile net income (loss) to net cash provided by operating activities:
Depreciation and amortization		334,589	292,768	[1]	284,969	[1]
Stock based compensation expense		13,105	10,580		7,568
Gain from early extinguishment of debt					(1,279)
Special items	[1]				465,649
Net increase (decrease) in deferred income taxes		99,139	(145,517)		(83,441)
Changes in operating assets and liabilities:
Decrease (increase) in restricted cash			8,243		(27)
Decrease (increase) in receivables		(21,464)	4,201		15,260
Decrease (increase) in income tax receivable		(20,620)	1,673		(4,118)
Increase in inventories		(7,935)	(1,246)		(1,986)
Decrease in other current assets		77,484	26,017		37,569
Decrease in deferred aircraft credits		(14,243)	(8,520)		(8,108)
Increase (decrease) in accounts payable and accrued aircraft rents		56,076	46,934		(47,563)
Increase in other current liabilities		6,031	20,084		3,758
NET CASH PROVIDED BY OPERATING ACTIVITIES		802,534	684,124		506,665
CASH FLOWS FROM INVESTING ACTIVITIES:
Purchases of marketable securities		(2,308,768)	(1,533,867)		(2,511,388)
Sales of marketable securities		2,451,344	1,440,283		2,388,168
Proceeds from the sale of aircraft, property and equipment			51,994		3,008
Proceeds from settlement of residual value guarantee aircraft agreements					90,000
Acquisition of property and equipment:
Aircraft and rotable spare parts		(1,062,380)	(661,176)		(1,138,963)
Deposits on aircraft		(41,937)	(46,733)		(650)
Buildings and ground equipment		(34,397)	(27,467)		(14,350)
Return of deposits applied towards acquired aircraft		49,550	36,533
Decrease (increase) in other assets		(36,816)	(10,904)		7,097
NET CASH USED IN INVESTING ACTIVITIES		(983,404)	(751,337)		(1,177,078)
CASH FLOWS FROM FINANCING ACTIVITIES:
Proceeds from issuance of long-term debt		784,665	471,677		926,069
Principal payments on long-term debt		(370,775)	(330,258)		(302,158)
Proceeds from issuance of common stock		5,212	4,660		8,142
Purchase of treasury stock		(54,448)	(20,008)
Employee income tax paid on vested equity awards		(13,556)	(5,080)
Decrease (increase) in debt issuance cost		(3,892)	(3,737)		(8,653)
Payment of cash dividends		(19,744)	(15,015)		(9,256)
NET CASH PROVIDED BY FINANCING ACTIVITIES		327,462	102,239		614,144
Increase (decrease) in cash and cash equivalents		146,592	35,026		(56,269)
Cash and cash equivalents at beginning of period		181,792	146,766		203,035
CASH AND CASH EQUIVALENTS AT END OF PERIOD		328,384	181,792		146,766
Non-cash investing activities:
Acquisition of rotable spare parts		367	755		5,688
Debt assumed on aircraft acquired off lease		59,132
Cash paid during the period for:
Interest, net of capitalized amounts		118,268	105,639		76,589
Income taxes		$ 2,661	$ 5,010		$ 1,212

[1] Amounts adjusted due to the adoption of Accounting Standards Update No. 2014-09, Revenue from Contracts with Customers (Topic 606). See Note 1 to the financial statements of this report for additional information.