Note Payable (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Debt Disclosure
Number of lines of credit	1	1
Aggregate borrowing capacity	$ 225	$ 225
Accordian feature available amount	$ 50	50
Line of credit facility, term	5 years
Expiration date of Line of credit	May 13, 2019
Note payable	$ 24	$ 20
Minimum
Debt Disclosure
Line of credit, interest rate	1.65%	1.31%
Maximum
Debt Disclosure
Line of credit, interest rate	2.45%	1.49%