Pension and Other Postretirement Benefits Benefit Obligation (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Award vesting period	3 years
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	$ 423,167
Balance, end of year	470,165	423,167
Paid from Company assets for unfunded nonqualified benefits	2,865	682
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	558,650	598,917
Service cost	22,205	24,119	22,215
Interest cost	26,817	23,914	24,896
Actuarial loss (gain)	56,333	(40,141)
Benefits paid from plan assets	(47,420)	(46,522)
Benefits paid from Company assets	(1,392)	(682)
Plan participants' contributions	0	0
Administrative expenses paid	(1,032)	(955)
Settlements	(1,473)	0
Benefit obligation, end of period	612,688	558,650	598,917
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	423,167	430,894
Actual return on plan assets	55,450	(250)
Employer contributions	42,865	40,682
Plan participants' contributions	0	0
Benefits paid	(48,812)	(47,204)
Administrative expenses paid	(1,032)	(955)
Settlements	(1,473)	0
Balance, end of year	470,165	423,167	430,894
Unfunded status	142,523	135,483
Accumulated benefit obligation	517,857	476,807
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Current accrued benefit cost	1,542	2,900
Non-current accrued benefit cost	140,981	132,583
Net amount recognized	142,523	135,483
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	143,734	138,186
Prior service cost (gain)	992	1,575
Accumulated other comprehensive loss	144,726	139,761
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract]
Net actuarial loss	12,032
Prior service cost (gain)	276
Accumulated other comprehensive loss	12,308
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	4.08%	4.87%
Rate of compensation increase	4.47%	4.25%
Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	76,134	83,836
Service cost	2,454	2,644	2,336
Interest cost	3,338	2,873	3,355
Actuarial loss (gain)	(102)	(8,766)
Benefits paid from plan assets	0	0
Benefits paid from Company assets	(6,057)	(5,855)
Plan participants' contributions	1,588	1,402
Administrative expenses paid	0	0
Settlements	0	0
Benefit obligation, end of period	77,355	76,134	83,836
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	4,469	4,453
Plan participants' contributions	1,588	1,402
Benefits paid	(6,057)	(5,855)
Administrative expenses paid	0	0
Settlements	0	0
Balance, end of year	0	0	0
Unfunded status	77,355	76,134
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Current accrued benefit cost	7,790	8,600
Non-current accrued benefit cost	69,565	67,534
Net amount recognized	77,355	76,134
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	12,944	13,780
Prior service cost (gain)	(5,193)	(6,525)
Accumulated other comprehensive loss	7,751	7,255
Pension and Other Postretirement Benefit Plans, Amounts that Will be Amortized from Accumulated Other Comprehensive Income (Loss) in Next Fiscal Year [Abstract]
Net actuarial loss	614
Prior service cost (gain)	(1,333)
Accumulated other comprehensive loss	$ (719)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	3.77%	4.34%
Rate of compensation increase	0.00%	0.00%
Health care cost trend rate, actuarial present value	7.50%	7.50%
Ultimate health care cost trend rate, actuarial present value	5.00%	5.00%
Health care cost trend rate, net periodic benefit cost	7.50%	8.00%	8.00%
Ultimate health care cost trend rate, net periodic benefit cost	5.00%	5.00%	5.00%
Year that rate reaches ultimate trend rate	2019