Employee benefit plans (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Unfunded, nonqualified benefit plan
Net periodic benefit cost for unfunded, nonqualified benefit plan	$ 2,000,000	$ 2,000,000	$ 6,000,000	$ 5,800,000
Pension Plans, Defined Benefit [Member]
Components of net periodic benefit cost:
Service cost	35,000	792,000	1,689,000	2,097,000
Interest cost	4,706,000	5,521,000	14,625,000	14,451,000
Expected return on assets	(5,679,000)	(6,373,000)	(17,106,000)	(17,057,000)
Amortization of prior service cost (credit)	(54,000)	42,000	33,000	111,000
Amortization of net actuarial (gain) loss	917,000	745,000	3,509,000	1,973,000
Amortization of net transition obligation	0	0	0	0
Curtailment Loss			1,218,000	0
Net periodic benefit cost, including amount capitalized	(75,000)	727,000	3,968,000	1,575,000
Less amount capitalized	323,000	268,000	858,000	651,000
Net periodic benefit cost	(398,000)	459,000	3,110,000	924,000
Other Postretirement Benefit Plans, Defined Benefit [Member]
Components of net periodic benefit cost:
Service cost	361,000	313,000	1,083,000	1,044,000
Interest cost	1,175,000	1,122,000	3,525,000	3,716,000
Expected return on assets	(1,263,000)	(1,261,000)	(3,789,000)	(4,230,000)
Amortization of prior service cost (credit)	(669,000)	(762,000)	(2,007,000)	(2,541,000)
Amortization of net actuarial (gain) loss	430,000	195,000	688,000	650,000
Amortization of net transition obligation	532,000	490,000	1,594,000	1,635,000
Curtailment Loss			0	0
Net periodic benefit cost, including amount capitalized	566,000	97,000	1,094,000	274,000
Less amount capitalized	(41,000)	(1,000)	(136,000)	83,000
Net periodic benefit cost	$ 607,000	$ 98,000	$ 1,230,000	$ 191,000