Postretirement Benefit Plans - Schedule of Assets at Fair Value for Defined-Benefit Pension Plans (Details) - Pension Benefits [Member] - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 355,053	$ 328,871
Percent of total pension assets	100.00%	100.00%
Exchange traded fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 147,357	$ 134,771
Percent of total pension assets	42.00%	41.00%
Other bonds [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 261	$ 258
Percent of total pension assets	0.00%	0.00%
Guaranteed annuity contract [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 19,254	$ 18,997
Percent of total pension assets	5.00%	6.00%
Other [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 5,281	$ 7,418
Percent of total pension assets	1.00%	2.00%
Corporate debt [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 158,227	$ 160,036
Percent of total pension assets	45.00%	49.00%
Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit pension plans-fair value of plan assets	$ 24,673	$ 7,391
Percent of total pension assets	7.00%	2.00%