Employee benefit plans (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Retired medical benefits
Current employee age of eligibility required by December 31, 2010 to receive retiree medical benefits (in years)			55
Number of years of continuous service required by December 31, 2010 to receive retiree medical benefits (in years)			10
Number of years of continuous service required after December 31, 2010 to receive retiree medical benefits (in years)			10
Current employee age of eligibility after December 31, 2010 to receive retiree medical benefits (in years)			60
Unfunded, nonqualified benefit plan
Age at which the defined benefit payments for unfunded, nonqualified plan begin following the employee's retirement (in years)			65
Period in which defined benefit payments for unfunded, nonqualified plan are made subsequent to retirement or death			15
Net periodic benefit cost for unfunded, nonqualified benefit plan	$ 1,900,000	$ 1,700,000	$ 4,000,000	$ 3,800,000
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost:
Service cost	827,000	501,000	1,654,000	1,305,000
Interest cost	4,959,000	4,004,000	9,919,000	8,930,000
Expected return on assets	(5,727,000)	(4,992,000)	(11,427,000)	(10,684,000)
Amortization of prior service cost (credit)	44,000	31,000	87,000	69,000
Amortization of net actuarial (gain) loss	1,049,000	256,000	2,592,000	1,228,000
Amortization of net transition obligation	0	0	0	0
Curtailment Loss	1,218,000	0	1,218,000	0
Net periodic benefit cost, including amount capitalized	2,370,000	(200,000)	4,043,000	848,000
Less amount capitalized	287,000	107,000	535,000	383,000
Net periodic benefit cost	2,083,000	(307,000)	3,508,000	465,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost:
Service cost	383,000	374,000	722,000	731,000
Interest cost	1,161,000	1,317,000	2,350,000	2,594,000
Expected return on assets	(1,308,000)	(1,577,000)	(2,526,000)	(2,969,000)
Amortization of prior service cost (credit)	(669,000)	(915,000)	(1,338,000)	(1,779,000)
Amortization of net actuarial (gain) loss	(53,000)	67,000	258,000	455,000
Amortization of net transition obligation	531,000	613,000	1,062,000	1,145,000
Curtailment Loss	0	0	0	0
Net periodic benefit cost, including amount capitalized	45,000	(121,000)	528,000	177,000
Less amount capitalized	(28,000)	37,000	(95,000)	84,000
Net periodic benefit cost	$ 73,000	$ (158,000)	$ 623,000	$ 93,000