Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	1 Months Ended	12 Months Ended
	Apr. 30, 2011	Dec. 31, 2011 Pension Plans [Member]	Dec. 31, 2010 Pension Plans [Member]	Dec. 31, 2009 Pension Plans [Member]	Dec. 31, 2011 Postretirement Plans [Member]	Dec. 31, 2010 Postretirement Plans [Member]	Dec. 31, 2009 Postretirement Plans [Member]
Change in Benefit Obligation
Projected benefit obligation at beginning of year		$ 761,384	$ 709,783		$ 133,717	$ 118,313
Service cost		20,762	19,217	18,638	4,789	4,265	3,912
Interest cost		42,383	41,621	41,941	6,450	6,651	7,045
Actuarial (gain) loss		81,699	27,094		(2,854)	11,730
Benefits paid		(38,854)	(36,331)		(7,176)	(7,242)
Projected benefit obligation at end of year		867,374	761,384	709,783	134,926	133,717	118,313
Change in Plan Assets
Fair value of assets at beginning of year		630,303	493,646		0	0
Actual return on plan assets	10,814	40,293	94,629		0	0
Employer contribution		4,906	78,359	27,616	7,176	7,242	6,455
Benefits paid		(38,854)	(36,331)		(7,176)	(7,242)
Fair value of assets at end of year		636,648	630,303	493,646	0	0	0
Funded status		(230,726)	(131,081)		(134,926)	(133,717)
Net amount recognized		(230,726)	(131,081)		(134,926)	(133,717)
Amounts Recognized in the Consolidated Balance Sheets
Noncurrent assets		0	1,083
Current liabilities		(4,880)	(5,028)		(9,966)	(9,100)
Noncurrent liabilities		(225,846)	(127,136)		(124,960)	(124,617)
Net amount recognized		(230,726)	(131,081)		(134,926)	(133,717)
Amounts Recognized in Accumulated Other Comprehensive Income
Net actuarial loss		281,352	202,135		26,006	30,008
Prior service cost		597	938		(4,141)	(4,815)
Total amount recognized		$ 281,949	$ 203,073		$ 21,865	$ 25,193