Investments (Allowance for Credit Losses and Recorded Investment in Commercial Mortgage and Other Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Credit Losses:
Individually evaluated for impairment	$ 0	$ 0
Collectively evaluated for impairment	1,794	1,558
Total ending balance	1,794	1,558	$ 2,651
Recorded Investment:
Individually evaluated for impairment	3,469	2,528
Collectively evaluated for impairment	1,081,744	1,149,411
Total ending balance	1,085,213	1,151,939
Commercial mortgage loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	1,728	1,513
Total ending balance	1,728	1,513	2,587
Recorded Investment:
Individually evaluated for impairment	2,316	2,528
Collectively evaluated for impairment	1,015,658	1,086,088
Total ending balance	1,017,974	1,088,616
Agricultural Property Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	66	45
Total ending balance	66	45	$ 64
Recorded Investment:
Individually evaluated for impairment	1,153	0
Collectively evaluated for impairment	66,086	63,323
Total ending balance	$ 67,239	$ 63,323