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Fair value measurements (Details 3) - USD ($)
$ in Thousands
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Concentration risks, percentage [Abstract]      
Percentage in fixed-income and other investments 49.00% 49.00% 51.00%
Percentage investment in common stock of large-cap companies 22.00% 23.00% 22.00%
Percentage investment in common stock of mid-cap companies 13.00% 14.00% 13.00%
Percentage investment in common stock of small-cap companies 11.00% 11.00% 11.00%
Percentage investment in target date investments 3.00% 2.00% 2.00%
Percentage investment in cash and cash equivalents 2.00% 1.00% 1.00%
Fair value, measurements, recurring [Member]      
Fair value measurements [Line Items]      
Assets, fair value disclosure $ 96,554 $ 94,774 $ 86,624
Fair value, measurements, recurring [Member] | Money market funds      
Fair value measurements [Line Items]      
Assets, fair value disclosure 9,085 6,965 2,551
Fair value, measurements, recurring [Member] | Insurance contract      
Fair value measurements [Line Items]      
Assets, fair value disclosure 76,941 [1] 77,388 [2] 73,775 [3]
Fair value, measurements, recurring [Member] | Mortgage backed securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure 10,063 10,217 9,885
Fair value, measurements, recurring [Member] | US Treasury securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure 465 204 413
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Money market funds      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Insurance contract      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 [1] 0 [2] 0 [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | Mortgage backed securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 1 | US Treasury securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2      
Fair value measurements [Line Items]      
Assets, fair value disclosure 96,554 94,774 86,624
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Money market funds      
Fair value measurements [Line Items]      
Assets, fair value disclosure 9,085 6,965 2,551
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Insurance contract      
Fair value measurements [Line Items]      
Assets, fair value disclosure 76,941 [1] 77,388 [2] 73,775 [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | Mortgage backed securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure 10,063 10,217 9,885
Fair value, measurements, recurring [Member] | Fair value, inputs, level 2 | US Treasury securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure 465 204 413
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Money market funds      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Insurance contract      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 [1] 0 [2] 0 [3]
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | Mortgage backed securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure 0 0 0
Fair value, measurements, recurring [Member] | Fair value, inputs, level 3 | US Treasury securities      
Fair value measurements [Line Items]      
Assets, fair value disclosure $ 0 $ 0 $ 0
[1] The insurance contract invests approximately 49 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 3 percent in target date investments and 2 percent in cash equivalents.
[2] The insurance contract invests approximately 49 percent in fixed-income investments, 23 percent in common stock of large-cap companies, 14 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.
[3] The insurance contract invests approximately 51 percent in fixed-income investments, 22 percent in common stock of large-cap companies, 13 percent in common stock of mid-cap companies, 11 percent in common stock of small-cap companies, 2 percent in target date investments and 1 percent in cash equivalents.