Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income taxes currently payable:
Federal										$ 49,944	$ 59,343	$ 64,227
State										6,120	5,675	5,181
Aggregate income taxes currently payable										56,064	65,018	69,408
Deferred income taxes (benefits):
Federal										(31,941)	3,830	(26,573)
State										960	880	(426)
Aggregate deferred income taxes										(30,981)	4,710	(26,999)
Total income tax expense		$ (4,123)	$ 11,739	$ 5,661	$ 11,806	$ 26,469	$ 18,631	$ 12,615	$ 12,013	$ 25,083	$ 69,728	$ 42,409
Statutory federal income tax rate (as a percent)	21.00%									35.00%	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										$ 47,328	$ 50,931	$ 24,891
State income taxes, net of federal income tax benefit										4,993	5,799	2,158
Tax contingencies reversed due to statute closings										(2,044)	(1,632)	(2,223)
Change in valuation allowances										(14,973)	2,130	5,174
Adjustments for Tax Act										(8,677)
Share-based compensation										(4,724)	(232)	165
Non-deductible HIF fees											10,204	9,953
Other-net										3,180	2,528	2,291
Total income tax expense		(4,123)	$ 11,739	$ 5,661	$ 11,806	26,469	$ 18,631	$ 12,615	$ 12,013	25,083	69,728	$ 42,409
Deferred tax assets:
Net operating loss carryforwards		13,384				19,727				13,384	19,727
Share-based compensation		9,639				14,704				9,639	14,704
Other accrued compensation		6,195				9,313				6,195	9,313
Claims reserves		4,527				6,251				4,527	6,251
Deferred revenue		2,606				4,986				2,606	4,986
Other non-deductible accrued liabilities		5,362				3,460				5,362	3,460
Amortization of goodwill and intangible assets						444					444
Indirect tax benefits		2,847				4,396				2,847	4,396
Other deferred tax assets		2,051				3,858				2,051	3,858
Total deferred tax assets		46,611				67,139				46,611	67,139
Valuation allowances		(2,368)				(17,117)				(2,368)	(17,117)
Deferred tax assets after valuation allowances		44,243				50,022				44,243	50,022
Deferred tax liabilities:
Depreciation		(22,906)				(41,311)				(22,906)	(41,311)
Amortization of goodwill and intangible assets		(29,501)								(29,501)
Other deferred tax liabilities		(3,321)				(5,586)				(3,321)	(5,586)
Total deferred tax liabilities		(55,728)				(46,897)				(55,728)	(46,897)
Net deferred tax assets						$ 3,125					$ 3,125
Net deferred tax liabilities		$ (11,485)								(11,485)
Provisional tax benefit recorded under the Tax Act										$ 8,200