Retirement Plans - Components of Net Periodic Benefits Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Pension Benefits [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	$ 3,230	$ 3,969	$ 9,690	$ 11,905
Interest cost	4,812	4,151	14,438	12,457
Expected return on plan assets	(8,056)	(6,982)	(24,167)	(20,946)
Amortization of net loss	7	1,605	20	4,813
Net periodic benefit cost	(7)	2,743	(19)	8,229
Other Post-retirement Benefits [Member]
Defined Benefit Plan Net Periodic Benefit Cost [Line Items]
Service cost	31	52	94	162
Interest cost	285	327	855	987
Expected return on plan assets
Amortization of net loss	91	459	273	1,320
Net periodic benefit cost	$ 407	$ 838	$ 1,222	$ 2,469