Investments - Troubled Debt Restructuring (Details) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2017 USD ($) loan	Dec. 31, 2016 loan
Private placement debt
Financing Receivable, Modifications [Line Items]
Troubled debt restructured loans, number \| loan	1	0
Pre-modification carrying value \| $	$ 3.3
Post-modification carrying value \| $	$ 3.3
Commercial mortgage
Financing Receivable, Modifications [Line Items]
Troubled debt restructured loans, number \| loan	0	0

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Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524

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Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

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