Income Taxes (Provision/Benefit for Income Taxes) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current [Abstract]
Federal	$ 98,288us-gaap_CurrentFederalTaxExpenseBenefit	$ 54,915us-gaap_CurrentFederalTaxExpenseBenefit	$ (3,824)us-gaap_CurrentFederalTaxExpenseBenefit
State	10,132	(163)	(1,039)
Total current	108,420us-gaap_CurrentIncomeTaxExpenseBenefit	54,752us-gaap_CurrentIncomeTaxExpenseBenefit	(4,863)us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred [Abstract]
Federal	(34,205)	12,699	53,642
State	(43,671)	(20,209)	26,859
Total deferred	(77,876)us-gaap_DeferredIncomeTaxExpenseBenefit	(7,510)us-gaap_DeferredIncomeTaxExpenseBenefit	80,501us-gaap_DeferredIncomeTaxExpenseBenefit
Total income tax expense	30,544us-gaap_IncomeTaxExpenseBenefit	47,242us-gaap_IncomeTaxExpenseBenefit	75,638us-gaap_IncomeTaxExpenseBenefit
Income taxes charged (credited) to shareholders' equity of Frontier [Abstract]
Utilization of the benefits arising from restricted stock	(472)	1,910	2,937
Deferred income taxes (benefits) arising from the recognition of additional pension/OPEB liability	(90,410)	132,432	(58,551)
Total income taxes charged (credited) to shareholders' equity of Frontier	(90,882)	134,342	(55,614)
Total income taxes	$ (60,338)	$ 181,584	$ 20,024

Amount of current federal tax expense (benefit) pertaining to income (loss) from continuing operations.

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