Special Charges and Unrealized (Gains) Losses on Investments - Components of Special Charges (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating:
CARES Act grant	$ (453,000)	$ (1,494,000)	$ (1,589,000)	$ 0					$ (3,536,000)	$ 0	$ 0
Severance and benefit costs	162,000	350,000	63,000	0	$ 2,000	$ 2,000	$ 6,000	$ 6,000	575,000	16,000	41,000
Impairment of assets	137,000	51,000	80,000	50,000	102,000	0	61,000	8,000	318,000	171,000	377,000
Termination of an engine maintenance service agreement									0	0	64,000
(Gains) losses on sale of assets and other special charges	5,000	12,000	(3,000)	13,000	26,000	25,000	4,000	4,000	27,000	59,000	5,000
Total operating special charges (credit)	(149,000)	(1,081,000)	(1,449,000)	63,000	130,000	27,000	71,000	18,000	(2,616,000)	246,000	487,000
Nonoperating credit loss on BRW Term Loan and related guarantee	0	0	0	697,000					697,000	0	0
Nonoperating special termination benefits and settlement losses	41,000	415,000	231,000	0					687,000	0	0
Nonoperating unrealized (gains) losses on investments	(101,000)	(15,000)	(9,000)	319,000	(81,000)	(21,000)	(34,000)	(17,000)	194,000	(153,000)	5,000
Total special charges and unrealized (gains) losses on investments	(60,000)	400,000	222,000	1,016,000	49,000	6,000	37,000	1,000	1,578,000	(153,000)	5,000
Total operating and nonoperating special charges (credit) and unrealized (gains) losses on investments	(209,000)	(681,000)	(1,227,000)	1,079,000					(1,038,000)	93,000	492,000
Income tax expense (benefit), net of valuation allowance	29,000	148,000	241,000	(14,000)	(11,000)	(2,000)	(8,000)	0	404,000	(21,000)	(110,000)
Income tax adjustments									0	0	(5,000)
Total operating and nonoperating special charges (credit) and unrealized (gains) losses on investments, net of income taxes	$ (180,000)	$ (533,000)	$ (986,000)	$ 1,065,000	$ 38,000	$ 4,000	$ 29,000	$ 1,000	$ (634,000)	$ 72,000	$ 377,000