Employee Benefit Plans - Schedule of plan assets by fair value hierarchy (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	$ 456,591	$ 472,417
Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	1,580	22,812
Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	435,477	431,839
Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	19,534	17,766
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	122,851	112,157
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	122,851	112,157
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
Equity securities, Other | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		11,783
Equity securities, Other | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		11,783
Equity securities, Other | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		0
Equity securities, Other | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		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	312,626	319,682
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	312,626	319,682
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
Fixed income, Other | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		7,725
Fixed income, Other | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		7,725
Fixed income, Other | Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		0
Fixed income, Other | Level 3
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets		0
Real Estate Fund | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	19,534	17,766
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	19,534	17,766
Cash equivalents | Estimate of fair value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	1,580	3,304
Cash equivalents | Level 1
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of plan assets	1,580	3,304
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