Investments (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Cash, cash equivalents and investments pledged as collateral	$ 5,853,744	$ 5,173,966
Investments held in trust	5,789,081	5,068,092
Investments pledged as collateral	$ 576,864	$ 442,184
Securities lending - Percent of market value required in collateral	102.00%

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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- References

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- Definition

The fair value of trading securities that serve as collateral for borrowings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5086-111524

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- Definition

Cash And Investments Pledged As Collateral

+ References

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- Definition

The required percentage of market value that is required as collateral for securities loaned pursuant to the Company's securities lending program. Collateral must be received in the form of cash, government securities and letters of credit and is held by a third party.

+ References

No definition available.

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