Employee Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Change in projected benefit obligation
Benefit obligation at the beginning of the period	$ 474,217	$ 414,452
Service cost	6,158	5,473
Interest cost	24,426	24,007
Actuarial (gain) loss	94,851	43,424
Regular benefits paid	(16,358)	(14,789)
Plan amendments		1,650
Benefit obligation at the end of the period	583,294	474,217
Accumulated benefit obligation	563,309	455,245
Change in plan assets
Value of plan assets at the beginning of the period	293,336	282,616
Actual return on plan assets	38,541	12,353
Employer contributions	21,015	13,156
Benefits paid	(16,358)	(14,789)
Value of plan assets at the end of the period	336,534	293,336
Under funded status at the end of the period	(246,760)	(180,881)
Defined benefit pension plan
Change in projected benefit obligation
Benefit obligation at the beginning of the period	401,134	349,435
Service cost	4,350	3,935	3,739
Interest cost	20,945	20,286	19,578
Actuarial (gain) loss	87,117	39,211
Regular benefits paid	(12,943)	(11,733)
Benefit obligation at the end of the period	500,603	401,134	349,435
Accumulated benefit obligation	491,712	393,487
Change in plan assets
Value of plan assets at the beginning of the period	293,336	282,616
Actual return on plan assets	38,541	12,353
Employer contributions	17,600	10,100
Benefits paid	(12,943)	(11,733)
Value of plan assets at the end of the period	336,534	293,336	282,616
Under funded status at the end of the period	(164,069)	(107,798)
Post-retirement medical plan
Change in projected benefit obligation
Benefit obligation at the beginning of the period	11,725	10,162
Service cost	817	622	728
Interest cost	569	585	713
Actuarial (gain) loss	974	693
Regular benefits paid	(623)	(337)
Benefit obligation at the end of the period	13,462	11,725	10,162
Accumulated benefit obligation	13,462	11,725
Change in plan assets
Employer contributions	623	337
Benefits paid	(623)	(337)
Under funded status at the end of the period	(13,462)	(11,725)
Supplemental executive retirement plan
Change in projected benefit obligation
Benefit obligation at the beginning of the period	61,358	54,855
Service cost	991	916	673
Interest cost	2,912	3,136	2,905
Actuarial (gain) loss	6,760	3,520
Regular benefits paid	(2,792)	(2,719)
Plan amendments		1,650
Benefit obligation at the end of the period	69,229	61,358	54,855
Accumulated benefit obligation	58,135	50,033
Change in plan assets
Employer contributions	2,792	2,719
Benefits paid	(2,792)	(2,719)
Under funded status at the end of the period	$ (69,229)	$ (61,358)