Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax expense computed at the statutory rate	$ 4,928	$ 9,670	$ 34,389
State income tax, net of federal benefit	838	1,485	3,114
Stock-based compensation expense	443	268	(380)
Change in U.S. tax rate	0	244	(18,921)
Decrease in taxes due to investment tax credit	(1,561)	(17,846)	(20,509)
Other	783	777	62
Total income tax expense (benefit)	$ 5,431	$ (5,402)	$ (2,245)