Pension Benefits (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Net periodic benefit cost:
Service cost	$ 287,000	$ 288,000
Interest cost	2,230,000	2,307,000
Expected return on plan assets	(3,069,000)	(2,746,000)
Amortization of net actuarial loss	202,000	166,000
Total net periodic cost	(350,000)	15,000
Weighted average assumptions used to determine net costs
Discount rate (as a percent)	3.85%	4.30%
Expected return on plan assets (as a percent)	7.50%	7.50%
Employer contributions	$ 1,500,000	$ 1,100,000