Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Components of Net Periodic Benefit Cost [Abstract]
Estimated future employer contributions in current fiscal year			$ 17,000
Pension and Supplemental Executive Retirement Plans [Member]
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,680	$ 2,203	5,128	$ 4,283
Interest Cost	4,016	3,985	7,924	7,994
Expected return on plan assets	(5,259)	(5,257)	(10,554)	(10,515)
Recognized net actuarial loss (gain)	1,533	250	2,742	541
Amortization of prior service cost	(211)	(423)	(422)	(465)
Net periodic benefit cost (benefit)	2,759	758	4,818	1,838
Other Postretirement Benefits [Member]
Components of Net Periodic Benefit Cost [Abstract]
Service cost	214	152	416	329
Interest Cost	171	144	349	327
Expected return on plan assets	(1,247)	(1,163)	(2,495)	(2,324)
Recognized net actuarial loss (gain)	(53)	(144)	(88)	(217)
Amortization of prior service cost	(1,663)	(1,662)	(3,325)	(3,325)
Net periodic benefit cost (benefit)	$ (2,578)	$ (2,673)	$ (5,143)	$ (5,210)