Investments (Fixed Maturity Securities Pledged And Received As Collateral) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	$ 21,966,803	$ 18,924,711
Available-for-sale Securities	23,210,968	19,619,084
Assets Held-in-trust	841,500	893,800
Reported Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Assets Held-in-trust	15,584,296	12,135,258
Reported Value Measurement [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	72,542	207,066
Estimate of Fair Value Measurement [Member]
Schedule of Available-for-sale Securities [Line Items]
Assets Held-in-trust	16,715,281	12,874,370
Estimate of Fair Value Measurement [Member] \| Securities Pledged as Collateral [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	75,622	210,676
Estimate of Fair Value Measurement [Member] \| Collateral Received Fixed Maturity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 590,417	$ 300,925

The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(b))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

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+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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No definition available.

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