Investments (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	$ 1,794	$ 1,558
Addition to (release of) allowance for losses	(131)	236
Charge-offs, net of recoveries	0	0
Total ending balance	1,663	1,794
Commercial Mortgage Loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	1,728	1,513
Addition to (release of) allowance for losses	(131)	215
Charge-offs, net of recoveries	0	0
Total ending balance	1,597	1,728
Agricultural Property Loans
Allowance for Loan and Lease Losses [Roll Forward]
Balance, beginning of year	66	45
Addition to (release of) allowance for losses	0	21
Charge-offs, net of recoveries	0	0
Total ending balance	$ 66	$ 66