Investments (Commercial Mortgage and Other Loans) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 1,085,213	$ 1,151,939
% of Total	100.00%	100.00%
Valuation allowance	$ (1,794)	$ (1,558)
Total Commercial mortgage and other loans	1,083,419	1,150,381
Apartments/Multi-Family
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 356,694	$ 318,667
% of Total	32.90%	27.70%
Hospitality
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 16,529	$ 54,597
% of Total	1.50%	4.70%
Industrial
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 180,619	$ 185,682
% of Total	16.70%	16.10%
Office
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 159,646	$ 161,980
% of Total	14.70%	14.10%
Other
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 99,119	$ 124,465
% of Total	9.10%	10.80%
Retail
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 205,367	$ 243,225
% of Total	18.90%	21.10%
Commercial mortgage loans
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 1,017,974	$ 1,088,616
% of Total	93.80%	94.50%
Agricultural property loans
Servicing Assets at Fair Value [Line Items]
Commercial mortgage and agricultural property loans	$ 67,239	$ 63,323
% of Total	6.20%	5.50%

Commercial mortgage loans, Percentage

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- Definition

The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 310
-Section 45
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82921503&loc=d3e21338-158488

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- Definition

Amount before allowance of mortgage loans on existing land or building. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5066-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(a)(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes loans held for sale. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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+ References

No definition available.

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