Investments (Allowance for Credit Losses and Recorded Investment) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Credit Losses:
Individually evaluated for impairment	$ 0	$ 0
Collectively evaluated for impairment	2,461	2,289
Total ending balance	2,461	2,289	$ 643
Recorded Investment:
Individually evaluated for impairment	1,644	1,715
Collectively evaluated for impairment	1,363,299	1,232,467
Total ending balance	1,364,943	1,234,182
Commercial Mortgage Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	2,407	2,267
Total ending balance	2,407	2,267	622
Recorded Investment:
Individually evaluated for impairment	1,644	1,715
Collectively evaluated for impairment	1,277,614	1,148,232
Total ending balance	1,279,258	1,149,947
Agricultural Property Loans
Allowance for Credit Losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	54	22
Total ending balance	54	22	$ 21
Recorded Investment:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	85,685	84,235
Total ending balance	$ 85,685	$ 84,235

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A valuation allowance for financing receivables that are expected to be uncollectible.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (h)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

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- Definition

Amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 14A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953550-111524

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