Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets
Allowance for doubtful accounts	$ 8,792	$ 10,735
Compensation and benefit-related accruals	50,936	70,199
Professional malpractice liability insurance	11,036	19,763
Deferred revenue	319	746
Net operating loss carryforwards	36,112	39,481
Stock options	6,591	9,533
Equity investments	1,452	1,567
Uncertain tax positions	503	499
Other	3,040	3,496
Deferred tax assets	118,781	156,019
Valuation allowance	(12,986)	(26,421)
Deferred tax assets, net of valuation allowance	105,795	129,598
Deferred tax liabilities
Deferred income	(19,608)	(26,068)
Investment in unconsolidated affiliates	(4,457)	(3,885)
Depreciation and amortization	(179,055)	(271,914)
Deferred financing costs	(4,528)	0
Other	(3,673)	(5,413)
Deferred tax liabilities	(211,321)	(307,280)
Deferred tax liabilities, net of deferred tax assets	(105,526)	(177,682)
Additional information related to valuation allowance
Other assets	19,391	21,396
Non-current deferred tax liability	(124,917)	$ (199,078)
Valuation allowance increase (release)	(13,400)
Valuation allowance release, federal net operating losses	14,100
Valuation allowance release, expired state net operating losses	200
Valuation allowance increase, newly generated state net operating losses	$ 900
Period of review for assessment of deferred tax assets	3 years