Pension Plan and Postretirement Benefits Other Than Pension (Details) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Amounts recognized in the statement of financial position:
Noncurrent liabilities	$ (45,936,000)	$ (13,333,000)
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Final number of years of employee's compensation to determine the benefits payable	10 years
Change in projected benefit obligation:
Net benefit obligation at beginning of year	172,105,000	184,165,000	148,412,000
Service cost	10,084,000	11,011,000	9,373,000
Interest cost	8,395,000	7,711,000	7,570,000
Benefits paid	(8,733,000)	(19,283,000)	(5,760,000)
Actuarial (gain) loss	26,410,000	(11,499,000)	24,570,000
Plan Amendment	(176,000)
Net benefit obligation at end of year	208,085,000	172,105,000	184,165,000
Accumulated benefit obligation	171,300,000	142,200,000
Change in plan assets:
Fair value of plan assets at beginning of year	170,430,000	133,911,000	103,404,000
Actual return on plan assets	(6,149,000)	38,802,000	21,267,000
Employer contributions	20,000,000	17,000,000	15,000,000
Benefits paid	(8,733,000)	(19,283,000)	(5,760,000)
Fair value of plan assets at end of year	175,548,000	170,430,000	133,911,000
Funded status at end of year	(32,537,000)	(1,675,000)	50,254,000
Amounts recognized in the statement of financial position:
Noncurrent liabilities	(32,537,000)	(1,675,000)	(50,254,000)
Net amount recognized at end of year	(32,537,000)	(1,675,000)	(50,254,000)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Transition obligation	(21,000)	(27,000)	(32,000)
Prior service cost	(1,179,000)	(1,822,000)	(2,377,000)
Accumulated gain (loss)	(75,681,000)	(30,602,000)	(74,286,000)
Accumulated other comprehensive income (loss)	(76,881,000)	(32,451,000)	(76,695,000)
Cumulative employer contributions in excess of net periodic benefit cost	44,344,000	30,776,000	26,441,000
Net amount recognized at end of year	(32,537,000)	(1,675,000)	(50,254,000)
Weighted average assumptions used to determine benefit obligation
Discount rate (as a percent)	4.13%	4.97%	4.22%
Rate of compensation increase (as a percent)	5.12%	5.12%	3.99%
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Age of employees after which the plan does not provide benefits	65 years
Change in projected benefit obligation:
Net benefit obligation at beginning of year	8,172,000	8,792,000	8,145,000
Service cost	719,000	788,000	693,000
Interest cost	397,000	361,000	400,000
Benefits paid	(527,000)	(283,000)	(560,000)
Actuarial (gain) loss	760,000	(1,807,000)	(223,000)
Retiree contributions	381,000	321,000	337,000
Net benefit obligation at end of year	9,902,000	8,172,000	8,792,000
Change in plan assets:
Employer contributions	146,000	(38,000)	223,000
Retiree contributions	381,000	321,000	337,000
Benefits paid	(527,000)	(283,000)	(560,000)
Funded status at end of year	(9,902,000)	(8,172,000)	(8,792,000)
Amounts recognized in the statement of financial position:
Current liabilities	(279,000)	(260,000)	(304,000)
Noncurrent liabilities	(9,623,000)	(7,912,000)	(8,488,000)
Net amount recognized at end of year	(9,902,000)	(8,172,000)	(8,792,000)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Prior service cost	(17,000)	(72,000)	(127,000)
Accumulated gain (loss)	265,000	1,041,000	(765,000)
Accumulated other comprehensive income (loss)	248,000	969,000	(892,000)
Cumulative employer contributions in excess of net periodic benefit cost	(10,150,000)	(9,141,000)	(7,900,000)
Net amount recognized at end of year	$ (9,902,000)	$ (8,172,000)	$ (8,792,000)
Weighted average assumptions used to determine benefit obligation
Discount rate (as a percent)	4.07%	4.94%	4.18%