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, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 10,942	$ 9,323	$ 7,096
Interest cost	4,050	3,671	3,249
Employer contributions	11,100
Plan amendments	672
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,832	1,354	1,089
Interest cost	398	252	195
Employer contributions	100
Change In Benefit Obligation [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at January 1	81,738	67,933
Service cost	10,942	9,323
Interest cost	4,050	3,671
Actuarial loss	14,981	6,967
Benefits paid	(6,951)	(6,156)
Plan amendments	672
Benefit obligation at December 31	105,432	81,738
Change In Benefit Obligation [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at January 1	6,793	4,955
Service cost	1,832	1,354
Interest cost	398	252
Actuarial loss	2,525	302
Benefits paid	(105)	(89)
Benefit obligation at December 31	11,464	6,793
Participant contributions	21	19
Change In Plan Assets [Member] | Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	11,084	12,513
Benefits paid	(6,951)	(6,156)
Fair value of plan assets at January 1	68,023	56,949
Fair value of plan assets at December 31	83,347	68,023
Actual return/(loss) on plan assets	11,191	4,717
Funded status of plans at December 31	(22,085)	(13,715)
Change In Plan Assets [Member] | Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	84	70
Benefits paid	(105)	(89)
Participant contributions	21	19
Funded status of plans at December 31	$ (11,464)	$ (6,793)