Pension and Other Post-Retirement Benefit Plans (Funded Status) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension Plans [Member]
Change in benefit obligation [Roll Forward]
Obligation, beginning balance	$ 791,182	$ 777,320
Service cost	0	0	$ 0
Interest cost	33,585	35,872	34,584
Plan transfers	2,643	1,226
Defined Benefit Plan, Benefit Obligation, Benefits Paid	33,779	33,593
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	(106,742)	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	106,742	0
Actuarial (gain) loss	45,411	10,357
Obligation, ending balance	740,783	791,182	777,320
Change in plan assets [Roll Forward]
Plan assets, beginning balance	715,885	672,153
Actual gain on plan assets	99,090	69,836
Employer contributions	4,476	6,263
Plan assets, ending balance	681,573	715,885	672,153
Funded status
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	8,483	0
Post-Retirement Healthcare Plans [Member]
Change in benefit obligation [Roll Forward]
Obligation, beginning balance	4,231	4,999
Service cost	158	123	177
Interest cost	142	170	192
Plan transfers	0	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	412	667
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	0	0
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement	0	560
Actuarial (gain) loss	(63)	166
Obligation, ending balance	4,056	4,231	4,999
Change in plan assets [Roll Forward]
Plan assets, beginning balance	0	0
Actual gain on plan assets	0	0
Employer contributions	412	667
Plan assets, ending balance	0	0	$ 0
Funded status
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Curtailment	0	0
Other Current Liabilities [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(1,497)	(1,357)
Other Current Liabilities [Member] | Post-Retirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(414)	(458)
Other Noncurrent Liabilities [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(57,713)	(73,940)
Other Noncurrent Liabilities [Member] | Post-Retirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ (3,642)	$ (3,773)
Non Pilot [Member] | Post-Retirement Healthcare Plans [Member]
Change in benefit obligation [Roll Forward]
Defined Benefit Plan, Benefit Obligation, (Increase) Decrease for Settlement				$ (600)