Benefit Payments Reflecting Expected Future Service as Appropriate which are Expected to be Paid in Future Years from Assets of Plans or by Company (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
2012	$ 28,688
2013	29,911
2014	31,237
2015	32,001
2016	32,738
2017-2021	185,062
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
2012	5,835
2013	6,072
2014	6,262
2015	6,449
2016	6,747
2017-2021	$ 38,516

Benefit Payments Reflecting Expected Future Service as Appropriate which are Expected to be Paid in Future Years from Assets of Plans or by Company (Detail) (USD $)
In Thousands, unless otherwise specified

Dec. 31, 2011

Pension Benefits

Defined Benefit Plan Disclosure [Line Items]

$ 28,688

29,911

31,237

32,001

32,738

185,062