Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred taxes on pension and OPEB costs, beginning balance	$ 288,712	$ 230,161	$ 227,284
Deferred taxes on pension and OPEB costs, other comprehensive income before reclassifications	293	(162)
Deferred taxes on pension and OPEB costs, amounts reclassified from accumulated other comprehensive loss	(4,095)	(2,715)
Deferred taxes on pension and OPEB costs, net current-period other comprehensive income (loss)	(3,802)	(2,877)
Deferred taxes on pension and OPEB costs, ending balance	284,910	227,284	288,712
Accumulated other comprehensive loss, beginning balance	(483,576)	(386,963)	(382,696)
Accumulated other comprehensive loss, other comprehensive (loss) before reclassifications	64	(162)
Accumulated other comprehensive loss, amounts reclassified from accumulated other comprehensive loss	6,682	4,429
Accumulated other comprehensive loss, net current-period other comprehensive income (loss)	6,746	4,267	(100,880)
Accumulated other comprehensive loss, ending balance	(476,830)	(382,696)	(483,576)
All Other [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated other comprehensive loss, beginning balance	(150)	(150)
Accumulated other comprehensive loss, other comprehensive (loss) before reclassifications	0	0
Accumulated other comprehensive loss, amounts reclassified from accumulated other comprehensive loss	0	0
Accumulated other comprehensive loss, net current-period other comprehensive income (loss)	0	0
Accumulated other comprehensive loss, ending balance	(150)	(150)
Pension Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	(697,874)	(575,163)
Other comprehensive income before reclassifications	(229)	0
Amounts reclassified from accumulated other comprehensive loss	10,074	7,736
Net current-period other comprehensive income (loss)	9,845	7,736
Balance, ending	(688,029)	(567,427)
Postretirement Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning	(74,264)	(41,811)
Other comprehensive income before reclassifications	0	0
Amounts reclassified from accumulated other comprehensive loss	703	(592)
Net current-period other comprehensive income (loss)	703	(592)
Balance, ending	$ (73,561)	$ (42,403)