Income Taxes - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Deferred tax assets, tax credit carryforwards, alternative minimum tax	$ 10,200,000		$ 13,500,000
Deferred tax assets, valuation allowance	4,500,000		5,100,000
Valuation allowance increase (decrease)	(600,000)
Unrecognized tax benefits	1,700,000		1,700,000
Unrecognized tax benefits, that would impact effective tax rate	0		0
Unrecognized tax benefits, period increase (decrease)	0	$ 0
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	0		0
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	404,100,000		$ 427,300,000
WindMark JV
Operating Loss Carryforwards [Line Items]
Federal tax refund	21,900,000
Tax year 2016
Operating Loss Carryforwards [Line Items]
Federal tax refund	$ 4,400,000