Pension plans and other postretirement benefits (Details 2) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Change in plan assets:
Fair value of plan assets at ending of year	$ 1,113,566,000	$ 834,904,000
Pension Benefits [Member]
Change in plan assets:
Fair value of plan assets at beginning of year	838,465,000	766,880,000
Actual return on plan assets	(6,635,000)	107,846,000
Employer contributions	75,600,000	4,504,000
Business combinations	259,250,000
Settlements		(3,231,000)
Benefits paid	(48,708,000)	(37,534,000)
Fair value of plan assets at ending of year	1,117,972,000	838,465,000
Other Postretirement Benefit Plans [Member]
Change in plan assets:
Fair value of plan assets at beginning of year	0	0
Employer contributions	6,859,000	4,846,000
Plan participants' contributions	3,383,000	2,776,000
Medicare Part D reimbursement	715,000	789,000
Benefits paid	(10,957,000)	(8,411,000)
Fair value of plan assets at ending of year	$ 0	$ 0