Employee Benefit Plans - Schedule of plan assets by fair value hierarchy (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	$ 407,797	$ 495,047
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	11,890	1,735
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	381,237	471,651
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	14,670	21,661
Equity securities, Common/collective trust funds | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	146,314	128,189
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	146,314	128,189
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 | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	234,923	343,462
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	234,923	343,462
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 | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	14,670	21,661
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	14,670	21,661
Cash equivalents | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	11,890	1,735
Cash equivalents | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	11,890	1,735
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