Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Components of Net Periodic Benefit Cost [Abstract]
Employer contributions in current fiscal year			$ 17,000
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,564	$ 2,142	7,692	$ 6,425
Interest cost	3,962	3,997	11,886	11,991
Expected return on plan assets	(5,278)	(5,258)	(15,832)	(15,773)
Recognized net actuarial loss (gain)	1,372	271	4,114	812
Amortization of prior service cost	(211)	(233)	(633)	(698)
Net periodic benefit cost (benefit)	2,409	919	7,227	2,757
Other Postretirement Benefits
Components of Net Periodic Benefit Cost [Abstract]
Service cost	208	165	624	494
Interest cost	174	164	523	491
Expected return on plan assets	(1,248)	(1,162)	(3,743)	(3,486)
Recognized net actuarial loss (gain)	(44)	(109)	(132)	(326)
Amortization of prior service cost	(1,661)	(1,663)	(4,986)	(4,988)
Net periodic benefit cost (benefit)	$ (2,571)	$ (2,605)	$ (7,714)	$ (7,815)