Income Taxes (Reconciliation of Provision for Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of Provision for Income Taxes
Federal income tax expense at statutory tax rate	$ 27,849	$ 53,892	$ 52,190
State income taxes, net of federal tax benefit	9,011	7,753	4,614
Impact of federal tax law change	211	(53,348)
Officers' compensation	3,078	2,622	3,807
Domestic production activities deduction		(2,668)	(4,018)
Noncontrolling interest	(3,232)	(2,137)
Federal R&D credits	(2,658)	(470)	(669)
Reversal of reserve for uncertain tax positions and taxes payable due to statue expirations	(1,958)	(4,337)
Other	2,531	(1,876)	(2,631)
Total tax expense (benefit)	$ 34,832	$ (569)	$ 53,293
Reconciliation of Effective Income Tax Rate
Federal income tax expense at statutory tax rate (in hundredths)	21.00%	35.00%	35.00%
State income taxes, net of federal tax benefit (in hundredths)	6.80%	5.00%	3.10%
Impact of federal tax law change (in hundredths)	0.20%	(34.60%)
Officers' compensation (in hundredths)	2.30%	1.70%	2.60%
Domestic production activities deduction (in hundredths)		(1.70%)	(2.70%)
Noncontrolling interest (in hundredths)	(2.40%)	(1.40%)
Federal R&D credits (in hundredths)	(2.00%)	(0.40%)	(0.50%)
Reversal of taxes payable due to statue expiration (in hundredths)	(1.50%)	(2.80%)
Other (in hundredths)	1.90%	(1.20%)	(1.80%)
Total (benefit) provision (in hundredths)	26.30%	(0.40%)	35.70%