Employee Benefit Plans - Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Changes in valuation methodology	no changes in the methodology	no changes in the methodology
Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	55,546	341,905	$ 320,699
Pension [Member] | Fair Value, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	15,149	110,676
Valuation methodology	Level 1 — Net asset value in an observable market.
Pension [Member] | Fair Value, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	40,397	231,229
Valuation methodology	Level 2 — Assets classified as level two are held in collective trust funds. The net asset value of a collective trust is calculated by determining the fair value of the fund’s underlying assets, deducting its liabilities, and dividing by the units outstanding as of the valuation date. These funds are not publicly traded; however, the unit price calculation is based on observable market inputs of the funds’ underlying assets.
Pension [Member] | Domestic equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	22,883	139,694
Pension [Member] | Domestic equity securities [Member] | Fair Value, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	4,557	29,293
Pension [Member] | Domestic equity securities [Member] | Fair Value, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	18,326	110,401
Pension [Member] | International equity securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	12,765	87,039
Pension [Member] | International equity securities [Member] | Fair Value, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	6,277	55,692
Pension [Member] | International equity securities [Member] | Fair Value, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	6,488	31,347
Pension [Member] | Domestic fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	14,643	85,222
Pension [Member] | Domestic fixed income securities [Member] | Fair Value, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	0	0
Pension [Member] | Domestic fixed income securities [Member] | Fair Value, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	14,643	85,222
Pension [Member] | International fixed income securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	2,428	15,134
Pension [Member] | International fixed income securities [Member] | Fair Value, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	2,428	15,134
Pension [Member] | International fixed income securities [Member] | Fair Value, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	0	0
Pension [Member] | Real estate fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	1,887	9,678
Pension [Member] | Real estate fund [Member] | Fair Value, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	1,887	9,678
Pension [Member] | Real estate fund [Member] | Fair Value, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	0	0
Pension [Member] | Short-term investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	940	5,138
Pension [Member] | Short-term investments [Member] | Fair Value, Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	0	879
Pension [Member] | Short-term investments [Member] | Fair Value, Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair Value	940	4,259