Postretirement Benefit Obligations - Defined Benefit Plans Disclosure (Details) - USD ($)	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in benefit obligation:
Benefit obligation at January 1, 2017				$ 33,887,000	$ 33,887,000
Service cost					7,629,000	$ 1,796,000	$ 0
Interest cost					1,333,000	315,000	0
Actuarial losses (gains)					8,190,000
Benefits Paid					(21,000)
Benefit obligation at December 31, 2017	$ 51,018,000				51,018,000	33,887,000
Change in plan assets:
Fair value of plan assets at January 1, 2017				0	0	0
Actual return on plan assets					592,000
Benefits paid					(21,000)
Company contributions during 2017	2,000,000	$ 11,000,000	$ 1,600,000	$ 2,200,000	16,750,000	0	0
Fair value of plan assets at December 31, 2017	17,321,000				17,321,000	$ 0	$ 0
Funded status of plan	33,697,000				33,697,000
Accrued pension liabilities-current	301,000				301,000
Accrued pension liabilities-noncurrent	33,396,000				33,396,000
Total pension liabilities recognized	$ 33,697,000				$ 33,697,000