Postretirement Benefits - Pension and SERP Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
PBO	$ 601,213	$ 556,199
Funded defined benefit pension \| Funded defined benefit pension \| Funded defined benefit pension
Defined Benefit Plan Disclosure [Line Items]
PBO	526,015	481,792
SERP \| SERP
Defined Benefit Plan Disclosure [Line Items]
PBO	$ 75,198	$ 74,407

Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanBenefitObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant