Income Taxes (Reconciliation of Provision for Income Taxes) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Amount
Federal income tax expense (benefit) at statutory tax rate	$ 36,599	$ (89,425)	$ 27,849
State income taxes, net of federal tax benefit	8,518	(18,442)	9,011
Stock based compensation	3,185	1,706	0
Impact of federal tax law changes	(14,476)	0	211
Officers' compensation	2,486	2,938	3,078
Goodwill impairment	0	43,990	0
Noncontrolling interests	(9,799)	(6,064)	(3,232)
Federal R&D credits	(3,007)	(3,998)	(2,658)
Reversal of reserve for uncertain tax positions due to statute expirations	(489)	(773)	(1,958)
Foreign tax rate differences	1,491	4,940	(19)
Other	(2,566)	(481)	2,550
Total expense (benefit):	$ 21,942	$ (65,609)	$ 34,832
Rate
Federal income tax expense (benefit) at statutory tax rate	21.00%	21.00%	21.00%
State income taxes, net of federal tax benefit	4.90%	4.30%	6.80%
Stock based compensation	1.80%	(0.40%)	0.00%
Impact of federal tax law changes	(8.30%)	0.00%	0.20%
Officers' compensation	1.40%	(0.70%)	2.30%
Goodwill impairment	0.00%	(10.30%)	0.00%
Noncontrolling interests	(5.60%)	1.40%	(2.40%)
Federal R&D credits	(1.70%)	0.90%	(2.00%)
Reversal of reserve for uncertain tax positions due to statute expirations	(0.30%)	0.20%	(1.50%)
Foreign tax rate differences	0.90%	(1.20%)	0.00%
Other	(1.50%)	0.20%	1.90%
Income tax expense (benefit)	12.60%	15.40%	26.30%