Estimated Fair Value of Investments Not Carried at Fair Value on Balance Sheet (Detail) - Estimate of Fair Value, Fair Value Disclosure - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]
Assets		$ 634,232	$ 747,854
Level 2
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]
Assets		608,183	632,834
Level 3
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]
Assets		26,049	115,020
Mortgage loans on real estate
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]
Assets	[1]	26,049	115,020
Mortgage loans on real estate | Level 3
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]
Assets	[1]	26,049	115,020
Policy loans
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]
Assets	[2]	608,183	632,834
Policy loans | Level 2
Fair Value, Estimate Not Practicable, Financial Statement Captions [Line Items]
Assets	[2]	$ 608,183	$ 632,834

[1] The fair value of mortgage loans on real estate is estimated by discounting the future cash flows using the current rates at which similar loans would be made to borrowers with similar credit ratings and/or similar remaining maturities.

[2] Policy loans are stated at unpaid principal balance. The book value of policy loans approximates their fair value.