Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income taxes currently payable:
Federal									$ 59,343	$ 64,227	$ 42,674
State									5,675	5,181	5,306
Aggregate income taxes currently payable									65,018	69,408	47,980
Deferred income taxes (benefits):
Federal									3,830	(26,573)	(3,236)
State									880	(426)	(1,055)
Aggregate deferred income taxes									4,710	(26,999)	(4,291)
Total income tax expense	$ 26,469	$ 18,631	$ 12,615	$ 12,013	$ 39,543	$ (7,254)	$ 5,987	$ 4,133	$ 69,728	$ 42,409	$ 43,689
Statutory federal income tax rate (as a percent)									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									$ 50,931	$ 24,891	$ 41,272
State income taxes, net of federal income tax benefit									5,799	2,158	2,738
Tax contingencies reversed due to statute closings									(1,632)	(2,223)	(17,318)
Change in valuation allowances									2,130	5,174	4,999
Non-deductible HIF fees									10,204	9,953	8,205
Other-net									2,296	2,456	3,793
Total income tax expense	26,469	$ 18,631	$ 12,615	$ 12,013	39,543	$ (7,254)	$ 5,987	$ 4,133	69,728	42,409	$ 43,689
Deferred tax assets:
Net operating loss carryforwards	19,727				20,686				19,727	20,686
Share-based compensation	14,704				12,320				14,704	12,320
Other accrued compensation	9,313				7,745				9,313	7,745
Claims reserves	6,251				8,243				6,251	8,243
Deferred revenue	4,986				3,644				4,986	3,644
Other non-deductible accrued liabilities	3,460				10,536				3,460	10,536
Amortization of goodwill and intangible assets	444				17,394				444	17,394
Indirect tax benefits	4,396				3,909				4,396	3,909
Other deferred tax assets	3,858				1,041				3,858	1,041
Total deferred tax assets	67,139				85,518				67,139	85,518
Valuation allowances	(17,117)				(15,458)				(17,117)	(15,458)
Deferred tax assets after valuation allowances	50,022				70,060				50,022	70,060
Deferred tax liabilities:
Depreciation	(41,311)				(39,843)				(41,311)	(39,843)
Other deferred tax liabilities	(5,586)				(3,381)				(5,586)	(3,381)
Total deferred tax liabilities	(46,897)				(43,224)				(46,897)	(43,224)
Net deferred tax assets	$ 3,125				$ 26,836				$ 3,125	$ 26,836