Postretirement Benefit Plans - Schedule of Defined-Benefit Plans Disclosures (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 324,243	$ 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	292	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	128,137	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	15,144	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	3,684	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	169,188	166,825
Percent of total pension assets	52.00%	52.00%
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 7,798	$ 9,038
Percent of total pension assets	2.00%	3.00%