Summary of Significant Accounting Policies - Adjustments to Amortization Expense (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Increase (decrease) to amortization expense:
Total	$ 881	$ (707)	$ 3,369
Retirement
Increase (decrease) to amortization expense:
Total	1,081	(313)	3,403
Life
Increase (decrease) to amortization expense:
Total	$ (200)	$ (394)	$ (34)

Amount of amortization expense (reversal of amortization expense) of deferred policy acquisition costs from a revision in estimated gross profit or estimated gross margin.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 30
-Section 35
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=95462302&loc=d3e10129-158409

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=35755530&loc=d3e11264-158415

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration