New Accounting Principles and Recent Accounting Pronouncements - Narrative (Details) - USD ($) $ in Thousands	Jan. 01, 2018	Dec. 31, 2020	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease assets		$ 927,658	$ 598,293	$ 618,100
Operating lease liabilities		103,758	92,286	155,500
Operating and finance lease liabilities, noncurrent				$ 468,400
Cumulative effect adjustment		4,394,147	5,804,611		$ 5,197,642
Retained Earnings
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		5,369,538	6,614,304		5,513,529	$ 5,245,242
Retained Earnings | Cumulative Effect, Period Of Adoption, Adjustment
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment					114,712
Retirement and Postretirement Benefit Plan Adjustments
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		(275,632)	(218,015)		(182,036)
Retirement and Postretirement Benefit Plan Adjustments | Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain tax effects from accumulated other comprehensive loss	$ 28,400
Deferred Loss on Interest Rate Derivative Hedges
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect adjustment		$ (1,690)	$ (2,894)		$ (7,899)
Deferred Loss on Interest Rate Derivative Hedges | Accounting Standards Update 2018-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of certain tax effects from accumulated other comprehensive loss	$ 1,800