Employee Benefit Plans (Details) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Pension Plans
Employee benefit plans
Employer contribution match, percentage of employee's contribution	50.00%
Total expense for all retirement plans	$ 4,100,000	$ 9,300,000	$ 7,000,000
Employee age required to participate in the plan	70 years 6 months
Components of net periodic pension cost
Service cost	4,551,000	7,431,000	5,713,000
Interest cost	5,203,000	5,729,000	5,452,000
Expected return on plan assets	(8,551,000)	(7,793,000)	(6,009,000)
Net amortization and deferral	1,356,000	2,600,000	516,000
Amortization of net loss	1,400,000
Net periodic benefit cost	2,559,000	7,967,000	5,672,000
Other changes in the defined benefit pension plans' plan assets and benefit obligations recognized in other comprehensive income
Net loss (gain)	(27,568,000)	11,167,000	29,319,000
Prior service cost (credit)	(14,000)	(26,919,000)
Amortization of prior service cost	2,321,000	160,000	160,000
Total recognized in other comprehensive income	(25,261,000)	(15,592,000)	29,479,000
Total recognized in net periodic pension cost and other comprehensive income/ (loss)	(22,702,000)	(7,625,000)	35,151,000
Pension Plans | Maximum
Employee benefit plans
Maximum employer contribution match, percentage of employee compensation	6.00%
Postretirement Healthcare Plan
Components of net periodic pension cost
Interest cost	63,000	66,000	86,000
Amortization of net loss	53,000	50,000	52,000
Net periodic benefit cost	116,000	116,000	138,000
Other changes in the defined benefit pension plans' plan assets and benefit obligations recognized in other comprehensive income
Net loss (gain)	(136,000)	64,000	(26,000)
Total recognized in other comprehensive income	(136,000)	64,000	(26,000)
Total recognized in net periodic pension cost and other comprehensive income/ (loss)	(20,000)	180,000	112,000
Estimated net loss for the postretirement healthcare benefit plan that will be amortized from accumulated other comprehensive income into net periodic benefit cost	$ 48,000
Postretirement Healthcare Plan | Minimum
Employee benefit plans
Employee age required to participate in the plan	55 years