Pension Plan and Postretirement Benefits Other Than Pension (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 7,101,000	$ 6,140,000	$ 5,276,000
Interest cost	7,195,000	6,596,000	6,387,000
Expected return on plan assets	(8,764,000)	(7,499,000)	(6,428,000)
Actuarial loss amortization	1,805,000	1,617,000	1,595,000
Prior service cost amortization	555,000	555,000	555,000
Transition obligation amortization	5,000	5,000	5,000
Net periodic benefit cost	7,897,000	7,414,000	7,390,000
Amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost in next fiscal year
Estimated net loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost	(4,100,000)
Estimated prior service cost that will be amortized from accumulated other comprehensive income into net periodic benefit cost	555,000
Estimated transition obligation that will be amortized from accumulated other comprehensive income into net periodic benefit cost	5,000
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	6.00%	6.25%	6.75%
Expected return on plan assets (as a percent)	7.75%	7.75%	7.75%
Rate of compensation increase (as a percent)	3.86%	3.86%
Expected rate of compensation increase in the year (as a percent)	3.86%	2.50%	0.00%
Expected benefit payments
2012	6,408,000
2013	7,823,000
2014	8,572,000
2015	8,034,000
2016	10,096,000
2017 through 2021	56,878,000
Total	97,811,000
Expected contribution in the next fiscal year	10,000,000
Pension Benefits | Maximum
Expected benefit payments
Expected contribution in the next fiscal year	20,000,000
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	558,000	443,000	371,000
Interest cost	402,000	364,000	343,000
Prior service cost amortization	55,000	45,000	39,000
Net periodic benefit cost	1,015,000	852,000	753,000
Amounts that will be amortized from accumulated other comprehensive income into net periodic benefit cost in next fiscal year
Estimated net loss that will be amortized from accumulated other comprehensive income into net periodic benefit cost	(12,000)
Estimated prior service cost that will be amortized from accumulated other comprehensive income into net periodic benefit cost	55,000
Weighted average assumptions used to determine net periodic benefit cost
Discount rate (as a percent)	6.00%	6.25%	6.75%
Expected benefit payments
2012	289,000
2013	364,000
2014	407,000
2015	424,000
2016	450,000
2017 through 2021	2,952,000
Total	4,886,000
Contributions by participants	$ 359,000	$ 237,000	$ 157,000