Borrowings (Loan Agreement) (Details)	Dec. 31, 2017 USD ($)
Line of Credit Facility [Line Items]
Maximum revolving secured credit line	$ 20,000,000
Borrowing rate under agreement, at period end	2.35%
Avaiable for borrowing under line of credit	$ 20,000,000.0
2018	16,000,000
2019	16,000,000
2020	16,000,000
Long-term Debt	48,000,000
Line of Credit | JPMorgan Chase Bank, N.A. | Term Loan
Line of Credit Facility [Line Items]
Maximum revolving secured credit line	80,000,000
Amount outstanding	$ 48,000,000.0