Pension Benefits (Details) (USD $)	12 Months Ended
	Dec. 31, 2013 bond	Dec. 31, 2012	Dec. 31, 2011
Change in benefit obligation:
Beginning balance	$ 235,502,000	$ 218,806,000	$ 183,048,000
Interest cost	8,836,000	9,226,000	9,741,000
Actuarial (gain) loss	(25,368,000)	14,616,000	32,385,000
Benefits paid	(7,157,000)	(7,146,000)	(6,368,000)
Benefit obligation at end of period	211,813,000	235,502,000	218,806,000
Change in fair value of plan assets:
Beginning balance	164,048,000	146,630,000	143,818,000
Actual return on assets	15,068,000	19,648,000	6,480,000
Employer contributions	6,000,000	6,075,000	3,750,000
Administrative fees	(1,253,000)	(1,159,000)	(1,050,000)
Benefits paid	(7,157,000)	(7,146,000)	(6,368,000)
Fair value of plan assets at end of period	176,706,000	164,048,000	146,630,000
Reconciliation of benefit plan funded status to the amounts recognized in the entity's consolidated balance sheets
Fair value of plan assets	176,706,000	164,048,000	146,630,000
Benefit obligation	(211,813,000)	(235,502,000)	(218,806,000)
Funded status (deficit)	(35,107,000)	(71,454,000)
Other long-term liabilities	(35,107,000)	(71,454,000)
Weighted average assumptions used to determine benefit obligations
Discount rate (as a percent)	4.70%	3.85%
Net periodic benefit cost:
Service cost	1,200,000	1,150,000	1,050,000
Interest cost	8,836,000	9,226,000	9,741,000
Expected return on plan assets	(12,258,000)	(10,982,000)	(10,662,000)
Amortization of net actuarial loss	761,000	666,000	0
Total net periodic benefit cost	(1,461,000)	60,000	129,000
Other comprehensive (loss) income:
Current year actuarial (loss) gain	28,885,000	(5,293,000)	(36,566,000)
Total other comprehensive (loss) income	28,885,000	(5,293,000)	(36,566,000)
Accumulated other comprehensive loss, net of tax	(29,100,000)	(46,500,000)
DefinedBenefitPlanAccumulatedOtherComprehensiveIncomeTax	(9,400,000)	2,100,000
Estimated amounts that will be amortized from accumulated other comprehensive loss into net periodic benefit cost
Estimated amount to be amortized from AOCI	$ 0
Weighted average assumptions used to determine net costs
Discount rate (as a percent)	3.85%	4.30%	5.40%
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.50%	7.50%	7.50%
Number of a high grade bonds considered when selecting discount rate	500
SFTP Benefit Plan
Pension Benefits
Normal retirement age (in years)	65 years
Early retirement age, low end of range (in years)	55 years
Early retirement age, high end of range (in years)	64 years
Attainment of credited service (in years)	10 years
Threshold age for reduction in early retirement benefit (in years)	62 years
Number of highest consecutive period of average compensation, used in plan benefit calculation (in years)	5 years
Period of average compensation, used in plan benefit calculation (in years)	10 years