Investments (Allowance for Credit Losses and Recorded Investment in Commercial Mortgage and Other Loans) (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Allowance for Credit Losses:
Individually evaluated for impairment	$ 0	$ 0
Collectively evaluated for impairment	1,895	643
Total ending balance	1,895	643
Recorded Investment:
Gross of reserves: individually evaluated for impairment	1,750	0
Gross of reserves: collectively evaluated for impairment	1,232,945	438,815
Total ending balance, gross of reserves	1,234,695	438,815
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	1,875	622
Total ending balance	1,875	622
Recorded Investment:
Gross of reserves: individually evaluated for impairment	1,750	0
Gross of reserves: collectively evaluated for impairment	1,153,413	386,149
Total ending balance, gross of reserves	1,155,163	386,149
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	20	21
Total ending balance	20	21
Recorded Investment:
Gross of reserves: individually evaluated for impairment	0	0
Gross of reserves: collectively evaluated for impairment	79,532	49,926
Total ending balance, gross of reserves	79,532	49,926
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	0	2,740
Total ending balance, gross of reserves	0	2,740
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