Postretirement Benefit Plans - Schedule of Defined-Benefit Plans Disclosures (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 307,452	$ 291,753
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	280	267
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	123,293	115,287
Percent of total pension assets	40.00%	38.00%
Guaranteed Annuity Contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	13,676	13,769
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	3,654	1,667
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	165,429	147,445
Percent of total pension assets	54.00%	51.00%
Short-term Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 1,120	$ 13,318
Percent of total pension assets	0.00%	5.00%