Medical Claims and Benefits Payable Components of Change in Medical Claims and Benefits Payable (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2014	Mar. 31, 2013	Dec. 31, 2013
Insurance Claims [Roll Forward]
Balances at beginning of period	$ 669,787	$ 494,530	$ 494,530
Components of medical care costs related to:
Current period	1,773,332	1,347,181	5,434,443
Prior periods	(50,904)	(58,427)	(52,779)
Total medical care costs	1,722,428	1,288,754	5,381,664
Change in non-risk provider payables	(28,560)	(7,638)	111,267
Payments for medical care costs related to:
Current period	1,172,672	948,820	4,932,195
Prior periods	371,442	335,681	385,479
Total paid	1,544,114	1,284,501	5,317,674
Balances at end of period	$ 819,541	$ 491,145	$ 669,787
Benefit from prior period as a percentage of:
Balance at beginning of period	7.60%	11.80%	10.70%
Premium revenue, trailing twelve months	0.80%	1.00%	0.90%
Medical care costs, trailing twelve months	0.90%	1.10%	1.00%