Income Taxes - Schedule of Deferred Tax Asset and Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Deferred Tax Assets
Net operating losses	$ 223,402	$ 141,961
Alternative minimum tax credits	228,693	227,719
Foreign tax credits	3,837	4,525
Incentive compensation	22,509	21,961
Deferred compensation	7,869	9,531
Post-retirement benefits	13,556	13,689
Other	2,905	5,526
Less valuation allowance	(4,965)	(4,779)
Total	497,806	420,133
Deferred Tax Liabilities
Properties and equipment	1,303,840	1,248,532
Equity method investments	1,202	0
Derivative instruments	0	50,750
Total	1,305,042	1,299,282
Net deferred tax liabilities	$ 807,236	$ 879,149