Investments - Narrative (Details) $ in Thousands	Jun. 30, 2018 USD ($) positions	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Percentage of Fixed Income Investment portfolio invested in below investment grade or not rated	3.50%	3.30%
Percentage of Gross Unrealized losses in Fixed income securities portfolio related to securities below investment grade or not rated	3.90%	4.20%
Pledged Assets Separately Reported, Other Debt Securities Available-for-sale or Held-for-investment	$ 17,000,000	$ 18,800,000
Available for sale securities, Gross unrealized losses	506,702	199,250
Excluding Life Funds Withheld Assets
Schedule of Available-for-sale Securities [Line Items]
Available for sale securities, Gross unrealized losses	$ 506,702	$ 199,250
Available-for-sale, Securities in Unrealized Loss Positions, Qualitative Disclosure, Number of Positions \| positions	4,673
Available-for-sale Securities, Qualitative Disclosure, Number of Positions \| positions	7,847

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563

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The fair value, as of the date of each statement of financial position presented, of other securities, classified as available for sale or held for investment, which are owned but transferred to serve as collateral for the payment of the related debt obligation, primarily a secured borrowing or repurchase agreement, and that are reclassified and separately reported in the statement of financial position because the transferee has the right by contract or custom to sell or re-pledge them.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Percentage of Fixed Income Investment portfolio at fair value invested in below investment grade or not rated as at the balance sheet date.

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Percentage of Gross Unrealized losses in Fixed income securities portfolio as at the balance sheet date related to securities that were below investment grade or not rated.

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