Pension plans and other postretirement benefits - Data Relating to Funding Position of Defined Benefit Plans (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at end of year	$ 1,404,138,000	$ 1,113,566,000
Pension benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	1,496,490,000	947,993,000
Service cost	29,549,000	28,828,000	19,670,000
Interest cost	62,037,000	58,545,000	47,905,000
Plan participants' contributions
Amendments
Actuarial (gain) loss	116,237,000	194,622,000
Settlements/curtailments	(320,000)
Business combinations		315,210,000
Medicare Part D reimbursement
Benefits paid	(59,426,000)	(48,708,000)
Benefit obligation at end of year	1,644,567,000	1,496,490,000	947,993,000
Fair value of plan assets at beginning of year	1,117,972,000	838,465,000
Actual return on plan assets	143,430,000	(6,635,000)
Employer contributions	207,115,000	75,600,000
Business combinations		259,250,000
Plan participants' contributions
Medicare Part D reimbursement
Settlements	(320,000)
Benefits paid	(59,426,000)	(48,708,000)
Fair value of plan assets at end of year	1,408,771,000	1,117,972,000	838,465,000
Funded status	(235,796,000)	(378,518,000)
Net prepaid asset		2,878,000
Accrued liabilities	(235,796,000)	(381,396,000)
Net loss	480,084,000	474,153,000
Net prior service cost	(23,455,000)	(30,014,000)
Pre-tax adjustment to AOCI	456,629,000	444,139,000
Taxes	(179,227,000)	(174,324,000)
Net adjustment to AOCI	277,402,000	269,815,000
Other postretirement benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	90,104,000	61,675,000
Service cost	668,000	535,000	383,000
Interest cost	3,737,000	3,761,000	3,130,000
Plan participants' contributions	3,765,000	3,383,000
Amendments	(13,313,000)
Actuarial (gain) loss	(182,000)	10,303,000
Settlements/curtailments
Business combinations		20,689,000
Medicare Part D reimbursement	918,000	715,000
Benefits paid	(10,731,000)	(10,957,000)
Benefit obligation at end of year	74,966,000	90,104,000	61,675,000
Fair value of plan assets at beginning of year
Actual return on plan assets
Employer contributions	6,048,000	6,859,000
Business combinations
Plan participants' contributions	3,765,000	3,383,000
Medicare Part D reimbursement	918,000	715,000
Settlements
Benefits paid	(10,731,000)	(10,957,000)
Fair value of plan assets at end of year
Funded status	(74,966,000)	(90,104,000)
Net prepaid asset
Accrued liabilities	(74,966,000)	(90,104,000)
Net loss	12,134,000	12,845,000
Net prior service cost	(13,173,000)	161,000
Pre-tax adjustment to AOCI	(1,039,000)	13,006,000
Taxes	408,000	(5,105,000)
Net adjustment to AOCI	$ (631,000)	$ 7,901,000