Income Taxes - Components of Income Tax Expense (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income taxes currently payable:
Federal									$ 64,227	$ 42,674	$ 37,691
State									5,181	5,306	3,445
Aggregate income taxes currently payable									69,408	47,980	41,136
Deferred income taxes (benefits):
Federal									(26,573)	(3,236)	(1,726)
State									(426)	(1,055)	514
Aggregate deferred income taxes									(26,999)	(4,291)	(1,212)
Total income tax expense	$ 39,543	$ (7,254)	$ 5,987	$ 4,133	$ 15,305	$ (3,490)	$ 6,261	$ 25,613	$ 42,409	$ 43,689	$ 39,924
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									$ 24,891	$ 41,272	$ 57,815
State income taxes, net of federal income tax benefit									2,158	2,738	4,412
Tax contingencies reversed due to statute closings									(2,223)	(17,318)	(25,299)
Change in valuation allowances									5,174	4,999	18
Non-deductible HIF fees									9,953	8,205
Other-net									2,456	3,793	2,978
Total income tax expense	39,543	$ (7,254)	$ 5,987	$ 4,133	15,305	$ (3,490)	$ 6,261	$ 25,613	42,409	43,689	$ 39,924
Deferred tax assets:
Net operating loss carryforwards	20,686				15,867				20,686	15,867
Share-based compensation	12,320				14,519				12,320	14,519
Other accrued compensation	7,745				8,332				7,745	8,332
Claims reserves	8,243				9,429				8,243	9,429
Other non-deductible accrued liabilities	14,180				10,009				14,180	10,009
Amortization of goodwill and intangible assets	17,394								17,394
Other deferred tax assets	4,950				6,181				4,950	6,181
Total deferred tax assets	85,518				64,337				85,518	64,337
Valuation allowances	(15,458)				(12,363)				(15,458)	(12,363)
Deferred tax assets after valuation allowances	70,060				51,974				70,060	51,974
Deferred tax liabilities:
Depreciation	(39,843)				(42,644)				(39,843)	(42,644)
Amortization of goodwill and intangible assets					(9,407)					(9,407)
Other deferred tax liabilities	(3,381)				(3,647)				(3,381)	(3,647)
Total deferred tax liabilities	(43,224)				(55,698)				(43,224)	(55,698)
Net deferred tax assets	$ 26,836								$ 26,836
Net deferred tax liabilities					$ (3,724)					$ (3,724)