Plan Assets Measured and Reported in Financial Statements at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 475,940	$ 456,445
Collective trust funds | US equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	174,027	174,735
Collective trust funds | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	166,432	164,998
Collective trust funds | International fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47,260	41,523
US government and US government agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36,531	31,434
US corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,628	21,171
International corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,700	4,996
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,561	8,741
Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,801	8,847
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	561	1,520
Level 1 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	561	1,520
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	475,379	454,925
Level 2 | Collective trust funds | US equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	174,027	174,735
Level 2 | Collective trust funds | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	166,432	164,998
Level 2 | Collective trust funds | International fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47,260	41,523
Level 2 | US government and US government agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36,531	31,434
Level 2 | US corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	24,628	21,171
Level 2 | International corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,700	4,996
Level 2 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,000	7,221
Level 2 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,801	$ 8,847