Fair Value, Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Assets [Abstract]
Limited partnerships	$ 70,806	$ 76,469
Carrying Value [Member]
Assets [Abstract]
Limited partnerships	70,806	76,469
Liabilities [Abstract]
Short-term borrowings	20,000	20,000
Fair Value [Member]
Assets [Abstract]
Limited partnerships	70,806	76,469
Liabilities [Abstract]
Short-term borrowings	20,000	20,000
Fair Value [Member] | Level 1 [Member]
Assets [Abstract]
Limited partnerships	0	0
Liabilities [Abstract]
Short-term borrowings	0	0
Fair Value [Member] | Level 2 [Member]
Assets [Abstract]
Limited partnerships	0	0
Liabilities [Abstract]
Short-term borrowings	20,000	20,000
Fair Value [Member] | Level 3 [Member]
Assets [Abstract]
Limited partnerships	70,806	76,469
Liabilities [Abstract]
Short-term borrowings	$ 0	$ 0