Investments (Allowance for Credit Losses and Recorded Investment in Commercial Mortgage and Other Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Allowance for Credit Losses:
Individually evaluated for impairment	$ 0	$ 0
Collectively evaluated for impairment	1,558	2,651
Total ending balance	1,558	2,651
Recorded Investment:
Gross of reserves: individually evaluated for impairment	2,528	287
Gross of reserves: collectively evaluated for impairment	1,149,411	1,660,599
Total ending balance, gross of reserves	1,151,939	1,660,886
Commercial mortgage and other loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	1,513	2,587
Total ending balance	1,513	2,587
Recorded Investment:
Gross of reserves: individually evaluated for impairment	2,528	0
Gross of reserves: collectively evaluated for impairment	1,086,088	1,545,853
Total ending balance, gross of reserves	1,088,616	1,545,853
Agricultural Property Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	45	64
Total ending balance	45	64
Recorded Investment:
Gross of reserves: individually evaluated for impairment	0	287
Gross of reserves: collectively evaluated for impairment	63,323	106,336
Total ending balance, gross of reserves	63,323	106,623
Uncollateralized loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	0	0
Total ending balance	0	0
Recorded Investment:
Gross of reserves: individually evaluated for impairment	0	0
Gross of reserves: collectively evaluated for impairment	0	8,410
Total ending balance, gross of reserves	0	8,410
Receivables Acquired with Deteriorated Credit Quality [Member]
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	0	0
Receivables Acquired with Deteriorated Credit Quality [Member] | Commercial mortgage and other loans
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	0	0
Receivables Acquired with Deteriorated Credit Quality [Member] | Agricultural Property Loans
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	0	0
Receivables Acquired with Deteriorated Credit Quality [Member] | Uncollateralized loans
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	$ 0	$ 0