Pension and Other Postretirement Benefits - Net Periodic Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	$ 6,604	$ 6,342	$ 19,812	$ 19,026
Interest cost	6,954	7,066	20,863	21,197
Expected return on plan assets	(7,658)	(6,754)	(22,973)	(20,262)
Amortization of: Net actuarial loss	5,376	4,791	16,127	14,373
Amortization of: Prior service cost (gain)	105	106	315	318
Net periodic benefit cost	11,381	11,551	34,144	34,652
Other Postretirement Benefit Plans, Defined Benefit
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Service cost	582	572	1,745	1,716
Interest cost	711	757	2,133	2,272
Expected return on plan assets	0	0	0	0
Amortization of: Net actuarial loss	197	177	591	531
Amortization of: Prior service cost (gain)	(545)	(545)	(1,635)	(1,635)
Net periodic benefit cost	$ 945	$ 961	$ 2,834	$ 2,884