Fair Value, Balance Sheet Grouping (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets [Abstract]
Limited partnerships	$ 64,369	$ 70,806
Carrying Value [Member]
Assets [Abstract]
Limited partnerships	64,369	70,806
Liabilities [Abstract]
Short-term borrowings	20,000	20,000
Fair Value [Member]
Assets [Abstract]
Limited partnerships	64,369	70,806
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	64,369	70,806
Liabilities [Abstract]
Short-term borrowings	$ 0	$ 0