Postretirement Benefit Plans - Schedule of Defined-Benefit Plans Disclosures (Detail) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 320,818	$ 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	$ 277	$ 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	$ 127,530	$ 127,568
Percent of total pension assets	40.00%	39.00%
Guaranteed Annuity Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 15,259	$ 15,027
Percent of total pension assets	5.00%	5.00%
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 3,063	$ 4,156
Percent of total pension assets	1.00%	1.00%
Corporate Bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 155,642	$ 166,825
Percent of total pension assets	48.00%	52.00%
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 19,047	$ 9,038
Percent of total pension assets	6.00%	3.00%