Investments (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	$ 2,650	$ 2,289
Addition to (release of) allowance for losses	275	361
Charge-offs, net of recoveries	0	0
Total ending balance	2,925	2,650
Commercial Mortgage Loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	2,616	2,267
Addition to (release of) allowance for losses	276	349
Charge-offs, net of recoveries	0	0
Total ending balance	2,892	2,616
Agricultural Property Loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	34	22
Addition to (release of) allowance for losses	(1)	12
Charge-offs, net of recoveries	0	0
Total ending balance	$ 33	$ 34