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Fair Value Measurements (Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Assets:    
Restricted Investments $ 75,409 $ 70,974
Municipal Bonds [Member]    
Assets:    
Investments in lieu of retainage 1,100 1,200
US Government Agencies Securities [Member]    
Assets:    
Restricted and other investments 38,300 33,800
Corporate Debt Securities [Member]    
Assets:    
Restricted and other investments 35,600 35,800
Fair value measured on a recurring basis    
Assets:    
Cash and cash equivalents 198,122 193,685
Restricted cash 5,956 8,416
Restricted investments 75,409 70,974
Investments in lieu of retainage 95,622 90,791
Total 375,109 363,866
Fair value measured on a recurring basis | Quoted prices in active markets (Level 1) [Member]    
Assets:    
Cash and cash equivalents 198,122 193,685
Restricted cash 5,956 8,416
Investments in lieu of retainage 94,519 89,572
Total 298,597 291,673
Fair value measured on a recurring basis | Significant other observable inputs (Level 2) [Member]    
Assets:    
Restricted investments 75,409 70,974
Investments in lieu of retainage 1,103 1,219
Total $ 76,512 $ 72,193
Maximum [Member]    
Assets:    
Restricted And other investments, term 5 years 5 years
Certificates of Deposit [Member]    
Assets:    
Restricted and other investments $ 1,500 $ 1,400
Money Market Funds [Member]    
Assets:    
Investments in lieu of retainage $ 94,500 $ 89,600