Income Taxes - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Operating Loss Carryforwards [Line Items]
Deferred tax assets, tax credit carryforwards, alternative minimum tax	$ 12,700,000		$ 13,500,000
Deferred tax assets, valuation allowance	4,800,000		5,100,000
Valuation allowance increase (decrease)	(300,000)
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
Income tax receivable	26,672,000		27,057,000
Federal
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	0		0
State
Operating Loss Carryforwards [Line Items]
Operating loss carryforwards	$ 421,000,000		427,300,000
WindMark JV
Operating Loss Carryforwards [Line Items]
Income tax receivable			$ 22,300,000