Postretirement Benefit Obligations - Defined Benefit Plans Disclosure (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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in benefit obligation:
Benefit obligation at January 1,				$ 69,281				$ 48,450				$ 51,018	$ 69,281	$ 48,450	$ 51,018
Service Cost													8,021	6,855	8,008
Interest Cost													2,175	2,084	1,875
Actuarial losses (gains)													10,507	12,364	(12,324)
Benefits Paid													(847)	(472)	(127)
Benefit obligation at December 31,	$ 89,137				$ 69,281				$ 48,450				89,137	69,281	48,450
Change in plan assets:
Fair value of plan assets at January 1,				35,979				26,789				17,321	35,979	26,789	17,321
Actual return on plan assets													5,212	5,462	(2,205)
Benefits paid													(847)	(472)	(127)
Company Contributions	6,400	$ 0	$ 0	$ 1,700	0	$ 3,700	$ 500	$ 0	0	$ 3,250	$ 6,600	$ 1,950	8,100	4,200	11,800
Fair value of plan assets at December 31,	48,444				35,979				26,789				48,444	35,979	26,789
Under-Funded status of plan	40,693				33,302				21,661				40,693	33,302	21,661
Accrued pension liabilities-current	1,525				892				581				1,525	892	581
Accrued pension liabilities-noncurrent	39,168				32,410				21,080				39,168	32,410	21,080
Total pension liabilities recognized	$ 40,693				$ 33,302				$ 21,661				$ 40,693	$ 33,302	$ 21,661