Summary of Significant Accounting Policies - Taxes, Health Care Reform, Cash, Restricted Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Taxes
The ownership percentage for which the entity files a consolidated federal income tax return, low end of range	80.00%
Health Care Reform
Health Insurer Fee (HIF) fiscal year annual fees paid during the period	$ 26,500	$ 21,400
Revenues recorded associated with the accrual for the reimbursement of the economic impact of the ACA fees from its customers	$ 45,400	36,500
Cash and Cash Equivalents
Maximum maturity period of short-term and highly liquid interest bearing investments at time of purchase	3 months
Excess capital and undistributed earnings included in cash and cash equivalents	$ 85,300
Restricted Assets
Restricted cash	133,597	215,325	$ 236,696
Restricted short-term investments	277,556	132,808
Restricted deposits (included in other current assets)	27,752	30,620
Restricted long-term investments	3,826	43,293
Total	$ 442,731	$ 422,046