Pension and Other Postretirement Benefits (Net Periodic Benefit Cost) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 1,149	$ 863	$ 3,551	$ 3,359
Interest cost	1,756	1,771	5,308	5,159
Expected return on plan assets	(2,179)	(2,072)	(6,877)	(7,198)
Amortization of prior service cost	7	6	20	18
Amortization of transition obligation	0	0	0	0
Amortization of net actuarial (gain) or loss	538	58	2,146	2,098
Net periodic postretirement benefit cost	1,271	626	4,148	3,436
Estimated contributions, next fiscal year					282
Contributions			212
Estimated contributions, current fiscal year	70		70
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	530	18	598	80
Interest cost	109	77	258	219
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	100	29	155	87
Amortization of transition obligation	0	0	0	0
Amortization of net actuarial (gain) or loss	(6)	26	103	98
Net periodic postretirement benefit cost	733	150	1,114	484
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	272	194	816	582
Interest cost	201	185	603	555
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	29	16	85	48
Amortization of transition obligation	28	28	85	84
Amortization of net actuarial (gain) or loss	16	0	47	0
Net periodic postretirement benefit cost	546	423	1,636	1,269
Estimated contributions, next fiscal year					615
Contributions			461
Estimated contributions, current fiscal year	$ 154		$ 154