Investment Securities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Investment securities
Fair Value	$ 5,065	$ 5,063
Cost	$ 5,126	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	$ 3,819	3,811
Cost	$ 3,842	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	$ 1,246	1,252
Cost	$ 1,284	$ 1,278
AAA rated | Ultra-Short-Term Bond Fund
Investment securities
Investment holdings grade rated (as a percent)	43.00%
AAA rated | Short-Term Bond Index Fund
Investment securities
Investment holdings grade rated (as a percent)	70.00%