Benefit Plans (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Pension and Supplemental Executive Retirement Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	$ 2,283	$ 2,564	$ 6,848	$ 7,692
Interest cost	3,976	3,962	11,929	11,886
Expected return on plan assets	(4,877)	(5,278)	(14,631)	(15,832)
Recognized net actuarial loss (gain)	1,464	1,372	4,392	4,114
Amortization of prior service cost	(172)	(211)	(515)	(633)
Net periodic benefit cost (benefit)	2,674	2,409	8,023	7,227
Estimated future employer contributions in current fiscal year			11,400
Company contributions			2,600
Other Postretirement Benefit Plans
Components of Net Periodic Benefit Cost [Abstract]
Service cost	188	208	564	624
Interest cost	176	174	528	523
Expected return on plan assets	(1,222)	(1,248)	(3,665)	(3,743)
Recognized net actuarial loss (gain)	0	(44)	0	(132)
Amortization of prior service cost	(1,662)	(1,661)	(4,987)	(4,986)
Net periodic benefit cost (benefit)	$ (2,520)	$ (2,571)	$ (7,560)	$ (7,714)