Income Taxes - Net Deferred Income Tax Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017
Deferred income tax assets:
Self-insurance accruals	$ 82,042		$ 75,198
Net operating loss carryforwards	414,143		171,053
Alternative minimum taxes (1)	0	$ 23,000	22,552
Accrued compensation and benefits	56,066		63,536
Federal benefit on state tax positions	11,027		11,950
Pension benefits	91,890		82,547
Miscellaneous other accruals	20,760		19,608
Deferred tax assets gross	675,928		446,444
Valuation allowance	(16,186)		(18,667)
Deferred tax assets net	659,742		427,777
Deferred income tax liabilities:
Property and equipment basis difference	(1,936,372)		(1,614,963)
Other	(13,940)		(16,857)
Deferred tax liabilities, gross	(1,950,312)		(1,631,820)
Net deferred income tax liability	(1,290,570)		(1,204,043)
Deferred Tax Assets Direct Financing Leases And Other Assets	$ 14,000		$ 7,000