Investments (Allowance for Credit Losses and Recorded Investment in Commercial Mortgage and Other Loans) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Allowance for Credit Losses:
Individually evaluated for impairment	$ 0	$ 0
Collectively evaluated for impairment	2,090	2,651
Total ending balance	2,090	2,651
Recorded Investment:
Gross of reserves: individually evaluated for impairment	287	287
Gross of reserves: collectively evaluated for impairment	1,643,745	1,660,599
Total ending balance, gross of reserves	1,644,032	1,660,886
Loans acquired with deteriorated credit quality
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	0	0
Commercial Mortgage Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	2,031	2,587
Total ending balance	2,031	2,587
Recorded Investment:
Gross of reserves: individually evaluated for impairment	0	0
Gross of reserves: collectively evaluated for impairment	1,532,136	1,545,853
Total ending balance, gross of reserves	1,532,136	1,545,853
Commercial Mortgage Loans | Loans acquired with deteriorated credit quality
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	0	0
Agricultural Property Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	59	64
Total ending balance	59	64
Recorded Investment:
Gross of reserves: individually evaluated for impairment	287	287
Gross of reserves: collectively evaluated for impairment	103,199	106,336
Total ending balance, gross of reserves	103,486	106,623
Agricultural Property Loans | Loans acquired with deteriorated credit quality
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	0	0
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	8,410	8,410
Total ending balance, gross of reserves	8,410	8,410
Uncollateralized Loans | Loans acquired with deteriorated credit quality
Allowance for Credit Losses:
Total ending balance	0	0
Recorded Investment:
Total ending balance, gross of reserves	$ 0	$ 0