Significant Accounting Policies (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other components of net period pension cost	$ 4,750	$ 5,082	$ 9,501	$ 10,164
Wages and benefits	154,660	130,801	305,713	259,362
Increase (decrease) to beginning and ending cash and cash equivalents			248,703	73,965
Reduced income tax expense	$ 400		5,600
Reclassification of excess tax benefits into operating activities			335,440	302,725
Reclassification of excess tax benefits out of financing activities			$ (42,318)	(202,736)
Accounting Standards Update 2016-18
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Increase (decrease) to beginning and ending cash and cash equivalents				5,000	$ 5,000
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other components of net period pension cost		5,100		10,200
Wages and benefits		$ (5,100)		(10,200)
Accounting Standards Update 2016-09, Excess Tax Benefit Component
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification of excess tax benefits into operating activities				17,300
Reclassification of excess tax benefits out of financing activities				$ (17,300)