Pension and Other Postretirement Benefits Benefit Obligation (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
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	$ 470,165
Balance, end of year	447,258	$ 470,165
Paid from Company assets for unfunded nonqualified benefits	1,537	2,865
Pension Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	612,688	558,650
Service cost	25,607	22,205	$ 24,119
Interest cost	25,203	26,817	23,914
Actuarial loss (gain)	21,731	56,333
Benefits paid from plan assets	(48,471)	(47,420)
Benefits paid from Company assets	(1,537)	(1,392)
Plan participants' contributions	0	0
Administrative expenses paid	(1,153)	(1,032)
Settlements	0	(1,473)
Benefit obligation, end of period	634,068	612,688	558,650
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Balance, beginning of year	470,165	423,167
Actual return on plan assets	(13,283)	55,450
Employer contributions	41,537	42,865
Plan participants' contributions	0	0
Benefits paid	(50,008)	(48,812)
Administrative expenses paid	(1,153)	(1,032)
Settlements	0	(1,473)
Balance, end of year	447,258	470,165	423,167
Unfunded status	186,810	142,523
Accumulated benefit obligation	551,662	517,857
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Current accrued benefit cost	1,599	1,542
Non-current accrued benefit cost	185,211	140,981
Net amount recognized	186,810	142,523
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	170,180	143,734
Prior service cost (gain)	716	992
Accumulated other comprehensive loss	170,896	$ 144,726
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	12,088
Prior service cost (gain)	259
Accumulated other comprehensive loss (income)	$ 12,347
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	4.48%	4.08%
Rate of compensation increase	4.44%	4.47%
Postretirement Benefits
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation, beginning of period	$ 77,355	$ 76,134
Service cost	2,505	2,454	2,644
Interest cost	2,925	3,338	2,873
Actuarial loss (gain)	(2,165)	(102)
Benefits paid from plan assets	0	0
Benefits paid from Company assets	(6,131)	(6,057)
Plan participants' contributions	1,448	1,588
Administrative expenses paid	0	0
Settlements	0	0
Benefit obligation, end of period	75,937	77,355	76,134
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	4,683	4,469
Plan participants' contributions	1,448	1,588
Benefits paid	(6,131)	(6,057)
Administrative expenses paid	0	0
Settlements	0	0
Balance, end of year	0	0	$ 0
Unfunded status	75,937	77,355
Pension and Other Postretirement Defined Benefit Plans, Liabilities [Abstract]
Current accrued benefit cost	5,634	7,790
Non-current accrued benefit cost	70,303	69,565
Net amount recognized	75,937	77,355
Defined Benefit Plan, Accumulated Other Comprehensive Income (Loss), after Tax [Abstract]
Net actuarial loss	10,983	12,944
Prior service cost (gain)	(3,860)	(5,193)
Accumulated other comprehensive loss	7,123	$ 7,751
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	481
Prior service cost (gain)	(1,333)
Accumulated other comprehensive loss (income)	$ (852)
Defined Benefit Plan, Weighted Average Assumptions Used in Calculating Benefit Obligation [Abstract]
Discount rate	4.09%	3.77%
Rate of compensation increase	0.00%	0.00%
Health care cost trend rate, actuarial present value	6.50%	7.50%
Ultimate health care cost trend rate, actuarial present value	5.00%	5.00%
Health care cost trend rate, net periodic benefit cost	7.00%	7.50%	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