Nature of Operations and Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) aircraft	Dec. 31, 2017 USD ($)		Dec. 31, 2016 USD ($)		Jan. 01, 2019 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of aircraft leased | aircraft									260
Accounts receivable	$ 64,194				$ 42,731				$ 64,194	$ 42,731
Income Statement:
Total operating revenues	803,489	$ 829,275	$ 805,515	$ 783,400	771,241	$ 812,673	$ 791,512	$ 747,166	3,221,679	3,122,592	[1]	$ 3,063,702	[1]
Salaries, wages and benefits									1,201,518	1,192,067	[1]	1,205,459	[1]
Other operating expenses									272,826	250,778	[1]	255,559	[1]
Total operating expenses									2,747,399	2,734,393	[1]	3,236,386	[1]
Operating income	121,502	$ 137,925	$ 126,678	$ 88,175	92,939	$ 112,369	$ 106,596	$ 76,295	474,280	388,199	[1]	(172,684)	[1]
ASSETS
Other long-term assets	104,844				65,341				104,844	65,341
LIABILITIES:
Other long-term liabilities	66,870				58,662				$ 66,870	58,662
Operating Leases, Number of Aircraft Leased | aircraft									260
Flying agreements
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable	$ 52,700				33,900				$ 52,700	33,900
Income Statement:
Total operating revenues									3,169,520	3,078,297	[1]	3,010,738	[1]
Airport customer service and other
Income Statement:
Total operating revenues									52,159	44,295	[1]	52,964	[1]
Aircraft fuel
Income Statement:
Total operating expenses									117,657	85,136	[1]	70,701	[1]
Airport related expenses
Income Statement:
Total operating expenses									109,605	118,374	[1]	122,141	[1]
Up-front contract reimbursements
Income Statement:
Total operating expenses									$ 2,000	1,500
ASU 2016-02 | Forecast | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liability														$ 500,000
Right-of-Use Asset														650,000
ASU 2016-02 | Forecast | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Lease liability														450,000
Right-of-Use Asset														$ 600,000
ASU 2014-09 | Previously Reported
Income Statement:
Total operating revenues										3,204,268		3,121,206
Salaries, wages and benefits										1,196,227		1,211,380
Other operating expenses										299,303		305,041
Total operating expenses										2,816,069		3,293,890
Operating income										388,199		(172,684)
ASSETS
Other long-term assets					49,220					49,220
LIABILITIES:
Other long-term liabilities					42,541					42,541
ASU 2014-09 | Previously Reported | Flying agreements
Income Statement:
Total operating revenues										3,126,708		3,051,414
ASU 2014-09 | Previously Reported | Airport customer service and other
Income Statement:
Total operating revenues										77,560		69,792
ASU 2014-09 | Previously Reported | Aircraft fuel
Income Statement:
Total operating expenses										162,653		122,284
ASU 2014-09 | Previously Reported | Airport related expenses
Income Statement:
Total operating expenses										69,848		72,659
ASU 2014-09 | Adjustments
Income Statement:
Total operating revenues										(81,676)		(57,504)
Salaries, wages and benefits										(4,160)		(5,921)
Other operating expenses										(48,525)		(49,482)
Total operating expenses										(81,676)		(57,504)
ASSETS
Other long-term assets					16,121					16,121
LIABILITIES:
Other long-term liabilities					$ 16,121					16,121
ASU 2014-09 | Adjustments | Flying agreements
Income Statement:
Total operating revenues										(48,411)		(40,676)
ASU 2014-09 | Adjustments | Airport customer service and other
Income Statement:
Total operating revenues										(33,265)		(16,828)
ASU 2014-09 | Adjustments | Aircraft fuel
Income Statement:
Total operating expenses										(77,517)		(51,583)
ASU 2014-09 | Adjustments | Airport related expenses
Income Statement:
Total operating expenses										$ 48,526		$ 49,482

[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.