Income Taxes (Schedule of Deferred Tax Assets and Liabilities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accrued insurance	$ 24,600	$ 31,100
Operating loss carryforwards and tax credits	54,900	34,900
Unrealized gains and losses	11,400	26,300
Compensation and benefits	7,500	24,100
Bad debt	1,200	4,700
Other	7,900	12,400
Valuation allowance	(40,500)	(21,400)
Total deferred tax assets	67,000	112,100
Deferred tax liabilities:
Property and equipment	104,100	126,600
Goodwill	56,700	72,800
Other intangible assets	28,700	32,900
Investments in unconsolidated entities	52,800	0
Other	11,000	15,300
Total deferred tax liabilities	271,500	278,700
Net deferred tax liabilities	(204,518)	(166,600)
Long-Term Contracts [Member]
Deferred tax liabilities:
Deferred income or gain	11,300	20,200
Equity Investee [Member]
Deferred tax liabilities:
Deferred income or gain	$ 6,900	$ 10,900