Pension and Other Postretirement Benefits (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 1,701	$ 1,766	$ 3,402	$ 3,532
Interest cost	3,316	3,070	6,664	6,140
Expected return on plan assets	(4,553)	(4,235)	(9,106)	(8,470)
Amortization of prior service cost	61	66	122	132
Amortization of net actuarial loss	550	2,434	1,192	4,868
Settlement loss	843		1,627
Net periodic benefit cost	1,918	3,101	3,901	6,202
Other Postretirement Benefits
Components of net periodic benefit cost:
Service cost	1,944	2,486	3,888	4,972
Interest cost	7,726	7,198	15,452	14,396
Amortization of prior service cost	307	307	614	614
Amortization of net actuarial loss	3,892	4,734	7,784	9,468
Net periodic benefit cost	$ 13,869	$ 14,725	$ 27,738	$ 29,450