Postretirement Benefit Plans - Schedule of Assets at Fair Value for Defined-Benefit Pension Plans (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 311,972	$ 322,898
Percent of total pension assets	100.00%	100.00%
Other fixed maturities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 279	$ 284
Percent of total pension assets	0.00%	0.00%
Equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 121,199	$ 127,568
Percent of total pension assets	39.00%	39.00%
Guaranteed annuity contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 16,973	$ 15,027
Percent of total pension assets	6.00%	5.00%
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 3,571	$ 4,156
Percent of total pension assets	1.00%	1.00%
Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 153,536	$ 166,825
Percent of total pension assets	49.00%	52.00%
Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 16,414	$ 9,038
Percent of total pension assets	5.00%	3.00%