Restricted Investments (Details) - USD ($)	6 Months Ended	12 Months Ended
Jun. 30, 2018	Dec. 31, 2017
Restricted Investments
Guaranteed interest rate on deposit (as a percent)	5.00%	5.00%
Restricted investments	$ 805,100,000	$ 882,909,000
Restricted investments, long-term	$ 585,111,000	$ 653,585,000

The interest rate for investments that have an interest rate. For fixed rate investments, this indicates the fixed interest rate. If the investment has a variable interest rate, the rate stated here may be the rate that is currently relevant. In this case the "Investment, Interest Rate is Current Rate for Variable Rate Investment Flag", "Investment, Interest Rate is Discount Rate at Purchase Flag", "Investment, Interest Rate Reflects Current Yield Flag" should be used. If one of these flags are not appropriate to explain the interest rate that is indicated, then a new flag should be added or an explanation should be included in "Investment, Additional Information".

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

Name:	invest_InvestmentInterestRate
Namespace Prefix:	invest_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration