Employee Benefit Plans (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined benefit pension plan
Components of net periodic cost (benefit):
Service cost	$ 1,260	$ 1,136	$ 2,519	$ 2,175
Interest cost	5,171	5,258	10,344	10,472
Expected return on plan assets	(6,029)	(5,462)	(12,058)	(10,889)
Amortization of unrecognized prior service cost (benefit)	13	13	26	26
Amortization of loss (gain) from earlier periods	3,895	2,524	7,789	4,370
Net periodic cost (benefit)	4,310	3,469	8,620	6,154
Payments to plan	2,025	10,056	2,025	13,300
Defined benefit pension plan | Forecasted contribution
Components of net periodic cost (benefit):
Expected Employer contribution for the year ended December 31, 2013			6,075
Post-retirement medical plan
Components of net periodic cost (benefit):
Service cost	233	225	465	408
Interest cost	123	150	246	284
Amortization of transition obligation				(825)
Amortization of unrecognized prior service cost (benefit)	(413)	(413)	(825)
Amortization of loss (gain) from earlier periods	(104)	(94)	(207)	(227)
Net periodic cost (benefit)	(161)	(132)	(321)	(360)
Payments to plan	129	63	282	237
Post-retirement medical plan | Forecasted contribution
Components of net periodic cost (benefit):
Expected Employer contribution for the year ended December 31, 2013			564
Supplemental executive retirement plan
Components of net periodic cost (benefit):
Service cost	250	562	501	492
Interest cost	637	724	1,274	1,448
Amortization of unrecognized prior service cost (benefit)	233	233	466	466
Amortization of loss (gain) from earlier periods	325	147	650	294
Net periodic cost (benefit)	1,445	1,666	2,891	2,700
Payments to plan	930	698	1,860	1,396
Supplemental executive retirement plan | Forecasted contribution
Components of net periodic cost (benefit):
Expected Employer contribution for the year ended December 31, 2013			$ 3,720