General form of registration statement for all companies including face-amount certificate companies

Summary of Significant Accounting Policies - Schedule of Cash and Available-For-Sale Securities' Cost, Gross Unrealized Losses and Fair Value by Significant Investment Category (Detail)

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Summary of Significant Accounting Policies - Schedule of Cash and Available-For-Sale Securities' Cost, Gross Unrealized Losses and Fair Value by Significant Investment Category (Detail) - USD ($)
$ in Thousands
Sep. 30, 2017
Dec. 31, 2016
Sep. 30, 2016
Dec. 31, 2015
Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]          
Cost $ 60,494        
Gross Unrealized Losses (62)        
Fair Value 60,432        
Cash and cash equivalents 20,762 $ 12,078 $ 7,626 $ 19,197 $ 18,747
Marketable securities 39,670        
Cash [Member]          
Schedule of Available-for-sale Securities [Line Items]          
Cost 13,711        
Fair Value 13,711        
Cash and cash equivalents 13,711        
Level 1 [Member] | Money Market Funds [Member]          
Schedule of Available-for-sale Securities [Line Items]          
Cost 3,057        
Fair Value 3,057        
Cash and cash equivalents 3,057        
Level 2 [Member] | Corporate Securities [Member]          
Schedule of Available-for-sale Securities [Line Items]          
Cost 43,726        
Gross Unrealized Losses (62)        
Fair Value 43,664        
Cash and cash equivalents 3,994        
Marketable securities $ 39,670