Investment Securities (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Investment securities
Fair Value	$ 5,088	$ 5,063
Cost	$ 5,153	5,103
Ultra-Short-Term Bond Fund
Investment securities
Target investment allocation (as a percent)	75.00%
Non-investment grade rated holdings (as a percent)	1.00%
Maturity period of mutual funds	1 year
Fair Value	$ 3,840	3,811
Cost	$ 3,863	3,825
Short-Term Bond Index Fund
Investment securities
Target investment allocation (as a percent)	25.00%
Non-investment grade rated holdings (as a percent)	0.00%
Maturity period of mutual funds	3 years
Fair Value	$ 1,248	1,252
Cost	$ 1,290	$ 1,278
AAA rated | Ultra-Short-Term Bond Fund
Investment securities
Investment holdings grade rated (as a percent)	45.00%
AAA rated | Short-Term Bond Index Fund
Investment securities
Investment holdings grade rated (as a percent)	70.00%