Postretirement Benefit Obligations - Defined Benefit Plans Disclosure (Details) - USD ($) $ in Thousands	3 Months Ended											12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Change in benefit obligation:
Benefit obligation at January 1,				$ 51,018				$ 33,887				$ 51,018	$ 33,887	$ 34,935
Service Cost												8,008	7,629	1,796
Interest Cost												1,875	1,333	315
Actuarial losses (gains)												(12,324)	8,190	(3,159)
Benefits Paid												(127)	(21)	0
Benefit obligation at December 31,	$ 48,450				$ 51,018				$ 33,887			48,450	51,018	33,887
Change in plan assets:
Fair value of plan assets at January 1,				17,321				0				17,321	0	0
Actual return on plan assets												(2,205)	592	0
Benefits paid												(127)	(21)	0
Company Contributions	0	$ 3,250	$ 6,600	$ 1,950	1,950	$ 11,050	$ 1,600	$ 2,150	0	$ 0	$ 0	11,800	16,750	0
Fair value of plan assets at December 31,	26,789				17,321				0			26,789	17,321	0
Funded status of plan	21,661				33,697				33,887			21,661	33,697	33,887
Accrued pension liabilities-current	581				301				343			581	301	343
Accrued pension liabilities-noncurrent	21,080				33,396				33,544			21,080	33,396	33,544
Total pension liabilities recognized	$ 21,661				$ 33,697				$ 33,887			$ 21,661	$ 33,697	$ 33,887