Components of Net Realized Investment Gains (Losses) (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Interest Rate Swap
Schedule of Available-for-sale Securities [Line Items]
Net interest settlements	$ 4.8	$ 5.4	$ 5.5

Amount of gain (loss) included in earnings for the period from the increase (decrease) in fair value of interest rate derivatives not designated as hedging instruments.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5618551-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=64837297&loc=SL5624171-113959

Name:	us-gaap_GainLossOnInterestRateDerivativeInstrumentsNotDesignatedAsHedgingInstruments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration