Condensed Consolidating Financial Information - Condensed Consolidating Statements of Cash Flows (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Condensed Consolidating Statements of Cash Flows
Net Cash Provided By (Used In) Operating Activities	$ 46,887	$ 150,680
Cash Flows From Investing Activities:
Net payments to affiliates	0	0
Additions to property and equipment, including pre-delivery deposits	(46,845)	(74,261)
Proceeds from the sale and sale leaseback of aircraft and aircraft related equipment		2,780
Purchases of investments	(48,133)	(71,454)
Sales of investments	80,218	137,286
Other	0	(6,275)
Net cash used in investing activities	(14,760)	(11,924)
Cash Flows From Financing Activities:
Long-term borrowings	235,000	0
Repayments of long-term debt and finance lease obligations	(25,320)	(24,354)
Dividend payments	(5,514)	(5,811)
Net payments from affiliates	0	0
Repurchases of common stock	(7,510)	(11,086)
Other	(1,230)	(982)
Net cash provided by (used in) financing activities	195,426	(42,233)
Net increase in cash and cash equivalents	227,553	96,523
Cash, cash equivalents, and restricted cash - Beginning of Period	373,056	268,577
Cash, cash equivalents, and restricted cash - End of Period	600,609	365,100
Eliminations
Condensed Consolidating Statements of Cash Flows
Net Cash Provided By (Used In) Operating Activities	0	0
Cash Flows From Investing Activities:
Net payments to affiliates	31,126	25,170
Additions to property and equipment, including pre-delivery deposits	0	0
Proceeds from the sale and sale leaseback of aircraft and aircraft related equipment		0
Purchases of investments	0	0
Sales of investments	0	0
Other		0
Net cash used in investing activities	31,126	25,170
Cash Flows From Financing Activities:
Long-term borrowings	0
Repayments of long-term debt and finance lease obligations	0	0
Dividend payments	0	0
Net payments from affiliates	(31,126)	(25,170)
Repurchases of common stock	0	0
Other	0	0
Net cash provided by (used in) financing activities	(31,126)	(25,170)
Net increase in cash and cash equivalents	0	0
Cash, cash equivalents, and restricted cash - Beginning of Period	0	0
Cash, cash equivalents, and restricted cash - End of Period	0	0
Parent Issuer / Guarantor | Reportable Legal Entities
Condensed Consolidating Statements of Cash Flows
Net Cash Provided By (Used In) Operating Activities	(976)	(670)
Cash Flows From Investing Activities:
Net payments to affiliates	(9,000)	(4,250)
Additions to property and equipment, including pre-delivery deposits	0	0
Proceeds from the sale and sale leaseback of aircraft and aircraft related equipment		0
Purchases of investments	0	0
Sales of investments	0	0
Other		0
Net cash used in investing activities	(9,000)	(4,250)
Cash Flows From Financing Activities:
Long-term borrowings	0
Repayments of long-term debt and finance lease obligations	0	0
Dividend payments	(5,514)	(5,811)
Net payments from affiliates	22,126	20,920
Repurchases of common stock	(7,510)	(11,086)
Other	0	0
Net cash provided by (used in) financing activities	9,102	4,023
Net increase in cash and cash equivalents	(874)	(897)
Cash, cash equivalents, and restricted cash - Beginning of Period	1,228	5,154
Cash, cash equivalents, and restricted cash - End of Period	354	4,257
Subsidiary Issuer / Guarantor | Reportable Legal Entities
Condensed Consolidating Statements of Cash Flows
Net Cash Provided By (Used In) Operating Activities	50,724	152,752
Cash Flows From Investing Activities:
Net payments to affiliates	(22,126)	(20,920)
Additions to property and equipment, including pre-delivery deposits	(40,616)	(71,978)
Proceeds from the sale and sale leaseback of aircraft and aircraft related equipment		2,780
Purchases of investments	(48,133)	(71,454)
Sales of investments	80,218	137,286
Other		(6,275)
Net cash used in investing activities	(30,657)	(30,561)
Cash Flows From Financing Activities:
Long-term borrowings	235,000
Repayments of long-term debt and finance lease obligations	(25,320)	(24,352)
Dividend payments	0	0
Net payments from affiliates	0	0
Repurchases of common stock	0	0
Other	(1,230)	(982)
Net cash provided by (used in) financing activities	208,450	(25,334)
Net increase in cash and cash equivalents	228,517	96,857
Cash, cash equivalents, and restricted cash - Beginning of Period	362,933	255,279
Cash, cash equivalents, and restricted cash - End of Period	591,450	352,136
Non-Guarantor Subsidiaries | Reportable Legal Entities
Condensed Consolidating Statements of Cash Flows
Net Cash Provided By (Used In) Operating Activities	(2,861)	(1,402)
Cash Flows From Investing Activities:
Net payments to affiliates	0	0
Additions to property and equipment, including pre-delivery deposits	(6,229)	(2,283)
Proceeds from the sale and sale leaseback of aircraft and aircraft related equipment		0
Purchases of investments	0	0
Sales of investments	0	0
Other		0
Net cash used in investing activities	(6,229)	(2,283)
Cash Flows From Financing Activities:
Long-term borrowings	0
Repayments of long-term debt and finance lease obligations	0	(2)
Dividend payments	0	0
Net payments from affiliates	9,000	4,250
Repurchases of common stock	0	0
Other	0	0
Net cash provided by (used in) financing activities	9,000	4,248
Net increase in cash and cash equivalents	(90)	563
Cash, cash equivalents, and restricted cash - Beginning of Period	8,895	8,144
Cash, cash equivalents, and restricted cash - End of Period	$ 8,805	$ 8,707