Employee Benefit Plans - Schedule of plan assets by fair value hierarchy (Details) - Estimate of fair value - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	$ 361,891	$ 464,441
Equity securities, Common/collective trust funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	168,547	234,061
Fixed income, Common/collective trust funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	166,079	204,453
Real Estate Fund
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	24,798	23,102
Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	$ 2,467	$ 2,825