Employee Benefit Plans - Schedule of plan assets by fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	$ 2,729	$ 11,890
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	388,084	381,237
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	21,646	14,670
Equity securities, Common/collective trust funds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Equity securities, Common/collective trust funds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	204,084	146,314
Equity securities, Common/collective trust funds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Fixed income, Common/collective trust funds | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Fixed income, Common/collective trust funds | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	184,000	234,923
Fixed income, Common/collective trust funds | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Real Estate Fund | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Real Estate Fund | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Real Estate Fund | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	21,646	14,670
Cash equivalents | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	2,729	11,890
Cash equivalents | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Cash equivalents | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	0	0
Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	412,459	407,797
Estimate of fair value | Equity securities, Common/collective trust funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	204,084	146,314
Estimate of fair value | Fixed income, Common/collective trust funds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	184,000	234,923
Estimate of fair value | Real Estate Fund
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	21,646	14,670
Estimate of fair value | Cash equivalents
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	$ 2,729	$ 11,890