Combined Funded Status of Plans and their Reconciliation with Related Amounts Recognized in Consolidated Financial Statements (Postretirement Plans) (Detail) (USD $)	1 Months Ended	12 Months Ended
	Apr. 30, 2011	Dec. 31, 2012 Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2011 Other Postretirement Benefit Plans, Defined Benefit	Dec. 31, 2010 Other Postretirement Benefit Plans, Defined Benefit
Change in Benefit Obligation
Projected benefit obligation at beginning of year		$ 134,926,000	$ 133,717,000
Service cost		4,409,000	4,789,000	4,265,000
Interest cost		5,851,000	6,450,000	6,651,000
Plan amendments		(38,414,000)	0
Actuarial (gain) loss		13,562,000	(2,854,000)
Benefits paid		(6,834,000)	(7,176,000)
Projected benefit obligation at end of year		113,500,000	134,926,000	133,717,000
Change in Plan Assets
Fair value of assets at beginning of year		0	0
Actual return on plan assets	10,814,000	0	0
Fair value of assets at end of year		0	0	0
Funded status		(113,500,000)	(134,926,000)
Net amount recognized		(113,500,000)	(134,926,000)
Amounts Recognized in the Consolidated Balance Sheets
Current liabilities		(10,366,000)	(9,966,000)
Noncurrent liabilities		(103,134,000)	(124,960,000)
Net amount recognized		(113,500,000)	(134,926,000)
Amounts Recognized in Accumulated Other Comprehensive Income
Net actuarial loss		38,221,000	26,006,000
Prior service credit		(41,182,000)	(4,141,000)
Total amount recognized		$ (2,961,000)	$ 21,865,000