Note Payable (Narrative) (Details)		12 Months Ended
	Feb. 04, 2019 USD ($) extension	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Disclosure
Number of lines of credit		1	1
Aggregate borrowing capacity		$ 225,000,000	$ 225,000,000
Accordian feature available amount		$ 50,000,000	50,000,000
Line of credit facility, term		5 years
Expiration date of Line of credit		May 13, 2019
Note payable		$ 32,000,000	$ 24,000,000
Minimum
Debt Disclosure
Line of credit, interest rate		2.45%	1.65%
Maximum
Debt Disclosure
Line of credit, interest rate		4.75%	2.45%
Subsequent event
Debt Disclosure
Aggregate borrowing capacity	$ 300,000,000
Line of credit facility, term	5 years
Accordion feature	$ 300,000,000
Number of one year extensions | extension	2
Ratio of debt-to-total capital maximum	0.35