Line items represent reporting concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 12
-Sentence Column A

Investments - Additional Information (Detail)	12 Months Ended
Investments - Additional Information (Detail)	Dec. 31, 2017 USD ($) Issue	Dec. 31, 2016 USD ($) Issue	Dec. 31, 2015 USD ($)
Investment [Line Items]
Securities, cash and short-term investments held on deposit	$ 657,000,000	$ 600,000,000
Securities, cash and short-term investments held on deposit, at fair value	753,000,000	663,000,000
Written down securities carried at fair value	$ 59,000,000
Written down securities carried at fair value, percentage	0.40%
Gross unrealized loss	$ (82,683,000)	$ (215,984,000)
Decrease in unrealized loss position	$ 133,000,000
Total issues of fixed-maturities \| Issue	1,502	1,565
Securities of state and municipal governments	7.00%
Percentage of invested assets rated below investment grade	4.00%
Par value of investment in fixed maturities rated below investment grade	$ 790,000,000
Amortized cost of investment in fixed maturities rated below investment grade	702,000,000
Fair value of investment in fixed maturities rated below investment grade	$ 679,000,000
TEXAS
Investment [Line Items]
State and municipal government securities at fair value invested by state	29.00%
OHIO
Investment [Line Items]
State and municipal government securities at fair value invested by state	9.00%
WASHINGTON
Investment [Line Items]
State and municipal government securities at fair value invested by state	8.00%
ILLINOIS
Investment [Line Items]
State and municipal government securities at fair value invested by state	7.00%
MICHIGAN
Investment [Line Items]
State and municipal government securities at fair value invested by state	5.00%
GEORGIA
Investment [Line Items]
State and municipal government securities at fair value invested by state	5.00%
OTHER STATES
Investment [Line Items]
Proportion of state and municipal government securities at fair value invested in selected states	5.00%
Fixed maturities available for sale
Investment [Line Items]
Other-than-temporary impairment	$ 245,000	$ 0	$ 0
Other-than-temporary impairment, net of tax	$ 159,000	$ 0	$ 0
Credit Concentration Risk
Investment [Line Items]
Concentration risk percentage	100.00%
Investment Portfolio \| Credit Concentration Risk \| Corporate Debt Securities and Redeemable Preferred Stock
Investment [Line Items]
Concentration risk percentage	85.00%