Employee Benefit Plan - Pension Plan Weighted-Average Asset Allocations (Details) - DB Pension Plan	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Weighted-average asset allocations	100.00%	100.00%
Equity securities
Defined Contribution Plan Disclosure [Line Items]
Weighted-average asset allocations	65.00%	68.00%
Debt securities
Defined Contribution Plan Disclosure [Line Items]
Weighted-average asset allocations	34.00%	30.00%
Other
Defined Contribution Plan Disclosure [Line Items]
Weighted-average asset allocations	1.00%	2.00%

Percentage of fair value of investments (categorized by debt securities, equity securities, real estate and other plan assets) to the fair value of plan assets held.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e2446-114920

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

Name:	us-gaap_DefinedBenefitPlanWeightedAverageAssetAllocations
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant