Stockholders' Equity:AOCI (Details) - USD ($) $ in Thousands		3 Months Ended	12 Months Ended
		Dec. 31, 2020	Dec. 31, 2020		Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance, net of taxes			$ 701,330		$ 672,590	$ 674,549
Net actuarial gain (loss), before taxes			6,832		(4,642)	14,178
Amortization of actuarial (gain) loss, before taxes	[1]		807		(175)	1,497
Effect of settlements, before taxes	[1]		6,250		1,342	23,255
Income taxes			35,281	[2]	879	(9,880)
Ending Balance, net of taxes		$ 850,799	850,799		701,330	672,590
Stranded tax effects of Plan Termination		38,800	38,794	[3]	0	0
Accumulated Other Comprehensive Loss [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance, net of taxes			(50,248)		(47,652)	(76,702)
Ending Balance, net of taxes		$ (1,078)	$ (1,078)		$ (50,248)	$ (47,652)

[1] Represents amounts reclassified from accumulated other comprehensive loss. These amounts are included in the computation of net periodic pension cost. For additional information, see Note 12, Retirement and Benefit Plans.

[2] For the year ended December 31, 2020, income taxes include $38.8 million related to the release of stranded tax effects upon Plan Termination, as discussed in Note 4, Income Taxes

[3] In December 2020, we eliminated our qualified defined benefit pension plan (Plan Termination), as discussed in Note 12, Retirement and Benefit Plans. Prior to the Plan Termination, "Accumulated other comprehensive loss" on our Consolidated Balance Sheet included the stranded tax effects of our conversion from a limited liability company to a corporation in 2013 and the adoption of the Tax Cuts and Jobs Act (the "Tax Act") in 2017. Upon Plan Termination, these stranded tax effects of $38.8 million were required to be released into income tax expense under GAAP.