Summary of Significant Accounting Policies - Additional Information (Detail) $ in Thousands		12 Months Ended
	Mar. 01, 2018 USD ($)	Dec. 31, 2017 USD ($) Country State PartnershipUnit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of countries in which company operates | Country		20
Number of states in which company licensed to sell insurance and annuities | State		49
Number of limited partnerships in which the company had investments | PartnershipUnit		1
Minimum percentage of fair value of loaned securities		102.00%
Impairment of intangible assets		$ 0	$ 0	$ 0
Reduced in net deferred tax asset		41,028
Subsequent Event
Summary Of Significant Accounting Policies [Line Items]
Cash dividends Payable, Date Declared	Mar. 01, 2018
Cash dividend declared	$ 200,000
Cash dividend payable, date to be paid	May 31, 2018
Loans 90 Days Past Due
Summary Of Significant Accounting Policies [Line Items]
Accrue interest		$ 0
Maximum
Summary Of Significant Accounting Policies [Line Items]
Percentage of value, impact of illiquidity premium to overall valuation		2.00%
Short-term investments with original maturities		3 months
Interest rates for future policy benefits liabilities		8.80%	8.80%
Minimum
Summary Of Significant Accounting Policies [Line Items]
Interest rates for future policy benefits liabilities		0.01%	0.01%