Benefit Plans (Details 8) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Benefits
Benefit Plans
Service cost	$ 2,025	$ 4,642	$ 3,727
Interest cost	10,992	12,212	11,602
Expected return on plan assets	(11,947)	(10,648)	(9,420)
Recognized net actuarial loss	4,930	8,445	6,450
Amortization of prior service cost (credit)	177	191	247
Settlement	2,302		212
Net periodic cost	8,479	14,842	12,818
Other Postretirement Benefits
Benefit Plans
Service cost	1,061	529	424
Interest cost	1,378	1,360	1,234
Recognized net actuarial loss	517	147
Amortization of prior service cost (credit)	(25)	(99)	(101)
Net periodic cost	$ 2,931	$ 1,937	$ 1,557