Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2013 Contract	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Commitment And Contingencies [Line Items]
Retention limits per life	$ 2,000,000
Percentage of insurance ceded on total life insurance in force	0.50%
Ratio of reinsurance ceded to premium income	0.30%
Life reinsurance assumed ratio to life insurance in force	2.50%
Reinsurance assumed ratio on premium income	0.90%	1.10%	1.20%
Rental expense for operating leases	4,100,000	3,600,000	4,800,000
Future minimum rental commitments in 2014	3,400,000
Future minimum rental commitments in 2015	3,300,000
Future minimum rental commitments in 2016	1,900,000
Future minimum rental commitments in 2017	1,100,000
Future minimum rental commitments in 2018	1,000,000
Future minimum rental commitments in aggregate	11,400,000
Investment in low-income housing interests	290,000,000	285,000,000
Obligations under commitments	58,000,000
Obligations under commitments due in 2014	41,000,000
Obligations under commitments due in 2015	8,000,000
Obligations under commitments due in 2016	0
Recorded unconditional purchase obligation thereafter	9,000,000
Commitment to purchase private placement Investment	33,800,000
Securities of state and municipal governments	10.00%
Proportion of state and municipal government securities at fair value invested in selected states	5.00%
Par value of investment in fixed maturities rated below investment grade	681,000,000
Amortized cost of investment in fixed maturities rated below investment grade	566,000,000
Fair value of investment in fixed maturities rated below investment grade	523,000,000
Percentage of invested assets rated below investment grade	4.00%
Number of guarantee agreements in place	3
Guaranty liability	0
Letter of credit maximum available amount				250,000,000
Letters of credit facility outstanding	198,000,000	198,000,000
Letter of Credit [Member]
Commitment And Contingencies [Line Items]
Letter of credit maximum available amount	250,000,000
Equipment Lease Guarantees [Member]
Commitment And Contingencies [Line Items]
Maximum exposure under guarantees	$ 6,500,000
Texas [Member]
Commitment And Contingencies [Line Items]
State and municipal government securities at fair value invested by state	(0.31)
Ohio [Member]
Commitment And Contingencies [Line Items]
State and municipal government securities at fair value invested by state	(0.07)
Washington [Member]
Commitment And Contingencies [Line Items]
State and municipal government securities at fair value invested by state	(0.07)
Illinois [Member]
Commitment And Contingencies [Line Items]
State and municipal government securities at fair value invested by state	(0.06)
Alabama [Member]
Commitment And Contingencies [Line Items]
State and municipal government securities at fair value invested by state	(0.05)