Summary of Significant Accounting Policies - Taxes, Health Care Reform, Cash, Restricted Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Health Care Reform
Health Insurer Fee (HIF) fiscal year annual fees paid during the period	$ 26,500	$ 26,500	$ 21,400
Revenues recorded associated with the accrual for the reimbursement of the economic impact of the ACA fees from its customers	44,000	45,400	36,500
Receivables related to terminated contract that customer has expressed their unwillingness to pay	$ 13,300
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 for regulated subsidiaries included in cash and cash equivalents	$ 117,700
Restricted Assets
Restricted cash and cash equivalents	81,776	133,597	$ 215,325
Restricted short-term investments	227,795	277,556
Restricted deposits (included in other current assets)	38,785	27,752
Restricted long-term investments	6,306	3,826
Total	$ 354,662	$ 442,731