Benefit Plans - Change in Project Benefit/Accumulated Benefit Obligations (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year		$ 379,324	$ 317,606
Company Service Cost		10,256	8,565	$ 11,338
Interest Cost		15,847	15,987	15,289
Plan Participants' Contributions		0	0
Net Actuarial (Gain)/Loss due to Assumption Changes		(24,118)	59,901
Net Actuarial (Gain)/Loss due to Plan Experience		7,155	(55)
Benefit Payments from Fund	[1]	(32,646)	(21,539)
Benefit Payments Directly by Company		(7,661)	(1,404)
Plan Amendments		19	(1)
Other Adjustment		1,307	264
Benefit Obligation at End of Year		349,483	379,324	317,606
Other Postretirement Benefit Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year		18,225	15,764
Company Service Cost		833	659	812
Interest Cost		697	653	618
Plan Participants' Contributions		361	336
Net Actuarial (Gain)/Loss due to Assumption Changes		(2,083)	2,276
Net Actuarial (Gain)/Loss due to Plan Experience		(397)	(855)
Benefit Payments from Fund	[1]	(1,147)	(645)
Benefit Payments Directly by Company		0	0
Plan Amendments		0	0
Other Adjustment		(66)	37
Benefit Obligation at End of Year		$ 16,423	$ 18,225	$ 15,764

[1] Includes lump sum payments of $22.4 million and $11.8 million in 2015 and 2014, respectively, from our pension plan to eligible participants, which were former employees with vested benefits.