General – Impact of Adoption of ASC 606 on Consolidated Balance Sheet (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets
Accounts receivable	$ 839,773		$ 660,775
Other current assets	119,164	$ 71,656	72,323
Total Current Assets	1,668,149	1,482,686	1,483,353
Other long-term assets	30,909	21,234	22,567
Total Assets	3,139,759	2,956,569	2,957,234
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	268,258	191,453	193,635
Other medical liabilities	183,222		177,002
Total Current Liabilities	1,023,096	890,121	892,303
Deferred credits and other long-term liabilities	35,346	24,844	19,100
Total Liabilities	1,820,989	1,684,302	1,680,740
Retained earnings	1,420,271	1,395,268	1,399,495
Total Stockholders' Equity	1,318,770	1,272,267	1,276,494
Total Liabilities and Stockholders' Equity	3,139,759	2,956,569	$ 2,957,234
ASU 2014-09 | Adjustments
Assets
Accounts receivable	(17,972)
Other current assets	(445)	(667)
Total Current Assets	(18,417)	(667)
Other long-term assets	9,199	(1,333)
Total Assets	(9,218)	(665)
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	(11,127)	(2,182)
Other medical liabilities	50
Total Current Liabilities	(11,077)	(2,182)
Deferred credits and other long-term liabilities	(6,835)	5,744
Total Liabilities	(17,912)	3,562
Retained earnings	8,694	(4,227)
Total Stockholders' Equity	8,694	(4,227)
Total Liabilities and Stockholders' Equity	(9,218)	$ (665)
ASU 2014-09 | Balance Without ASC 606 Adoption
Assets
Accounts receivable	821,801
Other current assets	118,719
Total Current Assets	1,649,732
Other long-term assets	40,108
Total Assets	3,130,541
LIABILITIES AND STOCKHOLDERS' EQUITY
Accrued liabilities	257,131
Other medical liabilities	183,272
Total Current Liabilities	1,012,019
Deferred credits and other long-term liabilities	28,511
Total Liabilities	1,803,077
Retained earnings	1,428,965
Total Stockholders' Equity	1,327,464
Total Liabilities and Stockholders' Equity	$ 3,130,541