Retirement and Employee Benefit Plans (Pension and Other Postretirement Benefit Costs) (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
Expected return on plan assets	(4,398)	(3,503)	(2,809)
Amortization of transition obligation	0	0	0
Amortization of prior service cost	344	346	334
Amortization of net loss	856	806	846
Net periodic benefit cost	9,796	7,994	6,393
Settlements and curtailments	0	223	0
Total benefit cost	9,796	8,217	6,393
Other Postretirement Benefits [Member]
Service cost	1,354	1,089	696
Interest cost	252	195	136
Expected return on plan assets	0	0	0
Amortization of transition obligation	64	65	65
Amortization of prior service cost	14	14	14
Amortization of net loss	11	21	7
Net periodic benefit cost	1,695	1,384	918
Settlements and curtailments	0	0	0
Total benefit cost	$ 1,695	$ 1,384	$ 918