Summary of Significant Accounting Policies - Health Care Reform, Cash and Restricted Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017
Health Care Reform
HIF fees	$ 26,500	$ 26,500
Cash and Cash Equivalents
Excess capital and undistributed earnings for regulated subsidiaries included in cash and cash equivalents			$ 143,900
Restricted Assets
Restricted cash and cash equivalents	81,776	$ 133,597	229,013
Restricted short-term investments	227,795		219,111
Restricted deposits (included in other current assets)	38,785		41,121
Restricted long-term investments	6,306		17,287
Total	$ 354,662		$ 506,532
Equity in restricted net assets of consolidated subsidiaries as a percentage of consolidated stockholders' equity			27.00%