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Income Taxes - Operating loss and credit carryforwards (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Alternative minimum tax credits
Operating loss carryforwards
Tax credit carryforwards	$ 2,140	$ 1,073
Federal
Operating loss carryforwards
Net operating loss carryforwards	63,964	116,315
Illinois
Operating loss carryforwards
Net operating loss carryforwards	$ 381,471	$ 420,764

X

- Definition

Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

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- Definition

The amount of the tax credit carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32559-109319

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