Commitments and Contingencies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) guarantee state lease	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Obligation with Joint and Several Liability Arrangement [Line Items]
Retention limits per life	$ 2,000,000
Percentage of insurance ceded on total life insurance in force (less than)	0.40%
Ratio of reinsurance ceded to premium income	0.20%
Life reinsurance assumed ratio to life insurance in force	1.80%
Reinsurance assumed ratio on premium income	0.70%	0.70%	0.80%
Rental expense	$ 6,446,000	$ 6,520,000	$ 6,722,000
Number of guarantee arrangements | guarantee	4
Guarantee liability	$ 0
Letters of credit facility outstanding	$ 177,000,000	$ 177,000,000
Number of states private entities contracted with to conduct audits | state	47
Letter of Credit
Obligation with Joint and Several Liability Arrangement [Line Items]
Letter of credit maximum available amount	$ 250,000,000
Equipment Lease Guarantees
Obligation with Joint and Several Liability Arrangement [Line Items]
Number of leases under guarantee | lease	3
Maximum exposure under guarantees	$ 10,000,000
Flight Equipment | Equipment Lease Guarantees
Obligation with Joint and Several Liability Arrangement [Line Items]
Number of leases under guarantee | lease	2
Software, Furniture and Equipment | Equipment Lease Guarantees
Obligation with Joint and Several Liability Arrangement [Line Items]
Number of leases under guarantee | lease	1
Commercial mortgage participations
Obligation with Joint and Several Liability Arrangement [Line Items]
Purchase commitment of private placement fixed maturities	$ 210,000,000
Insurance-related assessments
Obligation with Joint and Several Liability Arrangement [Line Items]
Guarantee liability	9,600,000
Guaranty Liabilities, Recoverable Amount	7,800,000
Guaranty liability, recoverable amount	$ 1,800,000