Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss, net of tax		$ (59,500)	$ (67,300)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	[1]		(12,800)
Net current period other comprehensive income ending balance		7,800	(10,500)	$ 7,800
Other Comprehensive Income (Loss), Defined Benefit Plan, Adjustment for Settlement or Curtailment Gain (Loss), Tax		0	0	0
Defined Benefit Pension Plan Adjustments
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss, net of tax		(59,500)	(67,300)	(44,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax			(16,000)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		5,100	5,500
Net current period other comprehensive income ending balance		2,700	(10,500)
Pension Plan [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement		0	0	0
Pension Plan [Member] | Defined Benefit Pension Plan Adjustments
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss, net of tax		(67,800)	(83,000)	(59,000)
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		9,800	(16,300)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		5,400	7,300
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	[1]		(15,000)
Net current period other comprehensive income ending balance		15,200	(9,000)
Other Postretirement Benefits Plan [Member]
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Accumulated other comprehensive loss, net of tax		8,300	15,700	15,000
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax		(7,100)	300
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		(300)	(1,800)
Tax Cuts and Jobs Act, Reclassification from AOCI to Retained Earnings, Tax Effect	[1]		2,200
Net current period other comprehensive income ending balance		(7,400)	(1,500)
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement		$ 0	$ 0	$ 0

[1] In February 2018, the FASB issued ASU No. 2018-2, Income Statement – Reporting Comprehensive Income (Topic 220), Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income, which allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the December 22, 2017, H.R. 1, Tax Cuts and Jobs Act (the Act). During 2018, we reclassified the income tax effects of the Act on pension and other postretirement employee benefits within accumulated other comprehensive loss to retained earnings.