Accumulated Other Comprehensive Income - Changes by Component (Details) - Jun. 30, 2015 - USD ($) $ in Thousands		Total	Total
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance			$ (37,043)
Ending balance		$ (34,854)	(34,854)
Parent Company [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(36,131)	(37,043)
Other comprehensive income (loss) before reclassifications		0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		1,277	2,189
Net current-period other comprehensive income (loss)		1,277	2,189
Ending balance		(34,854)	(34,854)
Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(166,811)	(170,957)
Other comprehensive income (loss) before reclassifications		0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		4,173	8,319
Net current-period other comprehensive income (loss)		4,173	8,319
Ending balance		(162,638)	(162,638)
Net unrealized gain (loss) and prior service cost on pension plans | Parent Company [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(36,131)	(36,710)
Other comprehensive income (loss) before reclassifications		0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		1,277	1,856
Net current-period other comprehensive income (loss)		1,277	1,856
Ending balance		(34,854)	(34,854)
Net unrealized gain (loss) and prior service cost on pension plans | Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(160,900)	(164,281)
Other comprehensive income (loss) before reclassifications		0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		4,094	7,475
Net current-period other comprehensive income (loss)		4,094	7,475
Ending balance		(156,806)	(156,806)
Energy Related Derivative [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Parent Company [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		0	(333)
Other comprehensive income (loss) before reclassifications		0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0	333
Net current-period other comprehensive income (loss)		0	333
Ending balance		0	0
Energy Related Derivative [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		0	(686)
Other comprehensive income (loss) before reclassifications		0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		0	686
Net current-period other comprehensive income (loss)		0	686
Ending balance		0	0
Interest Rate Swap [Member] | Accumulated Net Gain (Loss) from Designated or Qualifying Cash Flow Hedges [Member] | Subsidiaries [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning balance		(5,911)	(5,990)
Other comprehensive income (loss) before reclassifications		0	0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax		79	158
Net current-period other comprehensive income (loss)		79	158
Ending balance	[1]	$ (5,832)	$ (5,832)

[1] _(a) These AOCI components are included in the computation of net periodic pension cost (see Note 5 for additional details)