Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,304	$ 5,506	$ 12,804	$ 11,103
Interest cost	6,251	6,610	12,601	13,409
Expected return on plan assets	(7,123)	(6,637)	(14,148)	(13,312)
Amortization of: Net actuarial loss	5,090	4,285	10,014	8,819
Amortization of: Prior service cost (gain)	113	230	226	476
Settlement loss			0	789
Net periodic benefit cost	10,635	9,994	21,497	21,284
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	577	602	1,252	1,227
Interest cost	713	843	1,463	1,668
Expected return on plan assets	0	0	0	0
Amortization of: Net actuarial loss	271	359	522	634
Amortization of: Prior service cost (gain)	(545)	(540)	(1,090)	(1,090)
Settlement loss			0	0
Net periodic benefit cost	$ 1,016	$ 1,264	$ 2,147	$ 2,439