Investments (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	$ 2,289	$ 643
Addition to (release of) allowance for losses	361	1,646
Charge-offs, net of recoveries	0	0
Total ending balance	2,650	2,289
Commercial Mortgage Loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	2,267	622
Addition to (release of) allowance for losses	349	1,645
Charge-offs, net of recoveries	0	0
Total ending balance	2,616	2,267
Agricultural Property Loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	22	21
Addition to (release of) allowance for losses	12	1
Charge-offs, net of recoveries	0	0
Total ending balance	$ 34	$ 22