Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income taxes currently payable:
Federal									$ 13,593	$ 49,944	$ 59,343
State									7,145	6,120	5,675
Aggregate income taxes currently payable									20,738	56,064	65,018
Deferred income taxes (benefits):
Federal									307	(31,941)	3,830
State									(2,032)	960	880
Aggregate deferred income taxes									(1,725)	(30,981)	4,710
Total income tax expense	$ 448	$ 13,816	$ 4,824	$ (75)	$ (4,123)	$ 11,739	$ 5,661	$ 11,806	$ 19,013	$ 25,083	$ 69,728
Statutory federal income tax rate (as a percent)									21.00%	35.00%	35.00%
Differences between total income tax expense and amount computed using the statutory federal income tax rate
Income tax expense at federal statutory rate									$ 9,071	$ 47,328	$ 50,931
State income taxes, net of federal income tax benefit									2,968	4,993	5,799
State contingencies added									3,644	2,161	2,120
Tax contingencies reversed due to statute closings									(2,651)	(2,044)	(1,632)
Change in valuation allowances									(691)	(14,973)	2,130
Adjustments for Tax Act									335	(8,677)
Share-based compensation									(4,750)	(4,724)	(232)
Qualified research credit									(1,781)	(1,325)	(1,002)
Non-deductible executive compensation									3,052	104	34
Non-deductible HIF fees									8,246		10,204
Other-net									1,570	2,240	1,376
Total income tax expense	448	$ 13,816	$ 4,824	$ (75)	(4,123)	$ 11,739	$ 5,661	$ 11,806	19,013	25,083	$ 69,728
Deferred tax assets:
Net operating loss carryforwards	11,159				13,384				11,159	13,384
Share-based compensation	10,299				9,639				10,299	9,639
Other accrued compensation	5,508				6,195				5,508	6,195
Claims reserves	7,053				4,527				7,053	4,527
Deferred revenue	4,368				2,606				4,368	2,606
Other non-deductible accrued liabilities	6,057				5,362				6,057	5,362
Indirect tax benefits	3,296				2,847				3,296	2,847
Other deferred tax assets	2,130				2,051				2,130	2,051
Total deferred tax assets	49,870				46,611				49,870	46,611
Valuation allowances	(1,521)				(2,368)				(1,521)	(2,368)
Deferred tax assets after valuation allowances	48,349				44,243				48,349	44,243
Deferred tax liabilities:
Depreciation	(21,350)				(22,906)				(21,350)	(22,906)
Amortization of goodwill and intangible assets	(30,463)				(29,501)				(30,463)	(29,501)
Other deferred tax liabilities	(4,292)				(3,321)				(4,292)	(3,321)
Total deferred tax liabilities	(56,105)				(55,728)				(56,105)	(55,728)
Net deferred tax liabilities	$ (7,756)				$ (11,485)				$ (7,756)	(11,485)
Provisional tax benefit recorded under the Tax Act										$ 8,200