Retirement and Employee Benefit Plans (Changes in the Plans Benefit Obligations, Fair Value of Assets, and Funded Status) (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Benefits [Member]
Service cost	$ 9,323	$ 7,096	$ 5,148
Interest cost	3,671	3,249	2,874
Employer contributions	12,500
Pension Benefits [Member] | Change In Benefit Obligation [Member]
Benefit obligation at January 1	67,933	56,736
Service cost	9,323	7,096
Interest cost	3,671	3,249
Actuarial loss	6,967	6,311
Benefits paid	(6,156)	(5,029)
Plan amendments	0	168
Settlements	0	(598)
Benefit obligation at December 31	81,738	67,933
Participant contributions	0	0
Pension Benefits [Member] | Change In Plan Assets [Member]
Benefits paid	(6,156)	(5,029)
Fair value of plan assets at January 1	56,949	46,689
Actual return/(loss) on plan assets	4,717	6,276
Employer contributions	12,513	9,667
Participant contributions	0	0
Settlements	0	(654)
Fair value of plan assets at December 31	68,023	56,949
Funded status of plans at December 31	(13,715)	(10,984)
Other Postretirement Benefits [Member]
Service cost	1,354	1,089	696
Interest cost	252	195	136
Employer contributions	100
Other Postretirement Benefits [Member] | Change In Benefit Obligation [Member]
Benefit obligation at January 1	4,955	3,271
Service cost	1,354	1,089
Interest cost	252	195
Actuarial loss	302	462
Benefits paid	(89)	(77)
Plan amendments	0	0
Settlements	0	0
Benefit obligation at December 31	6,793	4,955
Participant contributions	19	15
Other Postretirement Benefits [Member] | Change In Plan Assets [Member]
Benefits paid	(89)	(77)
Fair value of plan assets at January 1	0	0
Actual return/(loss) on plan assets	0	0
Employer contributions	70	62
Participant contributions	19	15
Settlements	0	0
Fair value of plan assets at December 31	0	0
Funded status of plans at December 31	$ (6,793)	$ (4,955)