Employee Benefit Plans (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits
Benefit costs
Service cost			$ 1,488,000
Interest cost	8,844,000	8,344,000	10,008,000
Expected asset return	(12,664,000)	(11,860,000)	(11,491,000)
Amortization of net (gain) loss	135,000	628,000	429,000
Total benefit cost (income)	(3,685,000)	(2,888,000)	434,000
Actuarial assumptions:
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.25%	7.50%	7.75%
Expected return on plan assets (as a percent)	7.25%	7.50%	7.75%
Assumed health care cost trend rate, ultimate year	100,000
Pension Benefits | Minimum
Actuarial assumptions:
Discount rate (as a percent)	3.70%	4.40%
Discount rate (as a percent)	4.40%	3.55%	4.44%
Long-term rate of compensation increase (as a percent)			3.50%
Pension Benefits | Maximum
Actuarial assumptions:
Discount rate (as a percent)	3.74%	4.62%
Discount rate (as a percent)	4.62%	3.80%	4.59%
Long-term rate of compensation increase (as a percent)			5.00%
Other Postretirement Benefits
Benefit costs
Service cost	897,000	1,047,000	965,000
Interest cost	2,384,000	2,089,000	2,362,000
Amortization of net (gain) loss	60,000	356,000	118,000
Total benefit cost (income)	3,341,000	3,492,000	3,445,000
Actuarial assumptions:
Assumed health care cost trend rate, current (as a percent)	7.00%	7.50%	7.50%
Assumed health care cost trend rate, ultimate (as a percent)	5.00%	5.00%	5.00%
Assumed health care cost trend rate, ultimate year	$ 600,000
Other Postretirement Benefits | Minimum
Actuarial assumptions:
Discount rate (as a percent)	3.97%	4.75%
Discount rate (as a percent)	4.75%	3.85%	4.85%
Other Postretirement Benefits | Maximum
Actuarial assumptions:
Discount rate (as a percent)	4.15%	5.01%
Discount rate (as a percent)	5.01%	4.10%	5.09%