Investment Securities - Schedule of Amortized Cost and Estimated Fair Value of Fixed-Maturity Securities by Contractual Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Investments Debt And Equity Securities [Abstract]
Amortized Cost, 2018	$ 3,674
Total fixed-maturity securities, Amortized Cost	3,674	$ 76,912
Estimated Fair Value, 2018	3,660
Total fixed-maturity securities, Estimated Fair Value	$ 3,660	$ 76,831

Amount of investments in debt securities classified as trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26626-111562

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.4)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 3: 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.(a),1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

Name:	us-gaap_TradingSecuritiesDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant