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	1,225	2,651
Total ending balance	1,225	2,651
Recorded Investment:
Gross of reserves: individually evaluated for impairment	2,579	287
Gross of reserves: collectively evaluated for impairment	1,078,285	1,660,599
Total ending balance, gross of reserves	1,080,864	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	1,180	2,587
Total ending balance	1,180	2,587
Recorded Investment:
Gross of reserves: individually evaluated for impairment	2,579	0
Gross of reserves: collectively evaluated for impairment	1,014,496	1,545,853
Total ending balance, gross of reserves	1,017,075	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	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	63,789	106,336
Total ending balance, gross of reserves	63,789	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	0	8,410
Total ending balance, gross of reserves	0	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