Comprehensive Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Accumulated Other Comprehensive Income (Loss) [Line Items]
Deferred taxes on pension and OPEB costs, beginning balance			$ 288,712	$ 225,704	$ 230,161
Deferred taxes on pension and OPEB costs, other comprehensive income before reclassifications			356		973
Deferred taxes on pension and OPEB costs, amounts reclassified from accumulated other comprehensive loss			(8,190)		(5,430)
Deferred taxes on pension and OPEB costs, net current-period other comprehensive income (loss)			(7,834)		(4,457)
Deferred taxes on pension and OPEB costs, ending balance	280,878	225,704	280,878	288,712	225,704
Accumulated other comprehensive loss, beginning balance			(483,576)	(377,129)	(386,963)
Accumulated other comprehensive loss, other comprehensive (loss) before reclassifications			128		975
Accumulated other comprehensive loss, amounts reclassified from accumulated other comprehensive loss			13,364		8,859
Accumulated other comprehensive loss, net current-period other comprehensive income (loss)	6,746	5,567	13,492	(106,447)	9,834
Accumulated other comprehensive loss, ending balance	(470,084)	(377,129)	(470,084)	(483,576)	(377,129)
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			2		2
Accumulated other comprehensive loss, amounts reclassified from accumulated other comprehensive loss			0		0
Accumulated other comprehensive loss, net current-period other comprehensive income (loss)			2		2
Accumulated other comprehensive loss, ending balance	(148)	(148)	(148)		(148)
Pension Costs [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Balance, beginning			(697,874)		(575,163)
Other comprehensive income before reclassifications			(230)		0
Amounts reclassified from accumulated other comprehensive loss			20,148		15,473
Net current-period other comprehensive income (loss)			19,918		15,473
Balance, ending	(677,956)	(559,690)	(677,956)		(559,690)
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			1,406		(1,184)
Net current-period other comprehensive income (loss)			1,406		(1,184)
Balance, ending	$ (72,858)	$ (42,995)	$ (72,858)		$ (42,995)