Nature of Operations and Significant Accounting Policies - Contractholder and Policyholder Liabilities (Details) - Individual life insurance	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Contractholder and Policyholder Liabilities
Participating business as a percentage of life insurance in force (as a percent)	7.00%	8.00%	9.00%
Participating business as a percentage of life insurance policies in force (as a percent)	29.00%	33.00%	36.00%
Participating business as a percentage of life insurance premiums (as a percent)	30.00%	31.00%	36.00%

This percent places participating policies in the numerator and insurance contracts in force in the denominator. Participating insurance contracts allow policyholders to share in the experience of the entity, primarily through policy dividends.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.14(b)(1))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 50
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6483535&loc=d3e16392-158451

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant