General – Impact of Adoption of ASC 606 on Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable	$ 820,748		$ 660,775
Other current assets	102,829	$ 71,656	72,323
Total Current Assets	1,576,545	1,482,686	1,483,353
Other long-term assets	51,391	21,234	22,567
Total Assets	3,048,515	2,956,569	2,957,234
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	209,012	191,453	193,635
Other medical liabilities	162,009		177,002
Total Current Liabilities	918,571	890,121	892,303
Deferred credits and other long-term liabilities	34,959	24,844	19,100
Total Liabilities	1,716,764	1,684,302	1,680,740
Retained earnings	1,447,416	1,395,268	1,399,495
Total Stockholders' Equity	1,331,751	1,272,267	1,276,494
Total Liabilities and Stockholders' Equity	3,048,515	2,956,569	$ 2,957,234
ASU 2014-09 | Adjustments
Assets
Accounts receivable	(12,119)
Other current assets	1,116	(667)
Total Current Assets	(11,003)	(667)
Other long-term assets	4,667	(1,333)
Total Assets	(6,336)	(665)
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	(3,377)	(2,182)
Other medical liabilities	119
Total Current Liabilities	(3,258)	(2,182)
Deferred credits and other long-term liabilities	(7,380)	5,744
Total Liabilities	(10,638)	3,562
Retained earnings	4,302	(4,227)
Total Stockholders' Equity	4,302	(4,227)
Total Liabilities and Stockholders' Equity	(6,336)	$ (665)
ASU 2014-09 | Balance Without ASC 606 Adoption
Assets
Accounts receivable	808,629
Other current assets	103,945
Total Current Assets	1,565,542
Other long-term assets	56,058
Total Assets	3,042,179
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	205,635
Other medical liabilities	162,128
Total Current Liabilities	915,313
Deferred credits and other long-term liabilities	27,579
Total Liabilities	1,706,126
Retained earnings	1,451,718
Total Stockholders' Equity	1,336,053
Total Liabilities and Stockholders' Equity	$ 3,042,179