Pension and Other Post-retirement Benefit Plans (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pension Benefits
Components of net periodic benefit cost recognized
Service cost	$ 655,000	$ 645,000
Interest cost	2,355,000	2,755,000
Expected return on plan assets	(3,216,000)	(3,664,000)
Amortization of:
Prior service cost (benefit)	273,000	274,000
Actuarial loss	1,511,000	1,277,000
Net periodic benefit cost	1,578,000	1,287,000
Contributions by employer	200,000
Pension Benefits | Previously disclosed
Amortization of:
Anticipated contributions	800,000
Other Post-retirement Benefits
Components of net periodic benefit cost recognized
Service cost	558,000	508,000
Interest cost	821,000	1,039,000
Amortization of:
Prior service cost (benefit)	(763,000)	(770,000)
Actuarial loss	592,000	788,000
Curtailment		(634,000)
Net periodic benefit cost	1,208,000	931,000
Contributions by employer	1,100,000
Other Post-retirement Benefits | Previously disclosed
Amortization of:
Anticipated contributions	4,700,000
Other Post-retirement Benefits | Plan Expectation
Amortization of:
Anticipated contributions	$ 4,400,000