Pension Plan and Postretirement Benefits Other Than Pension (Details) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Amounts recognized in the statement of financial position:
Noncurrent liabilities	$ (62,458,000)	$ (56,548,000)
Pension Benefits
Pension Plan and Postretirement Benefits Other than Pension
Final number of years of employees compensation to determine the benefits payable	10 years
Change in projected benefit obligation:
Net benefit obligation at beginning of year	148,412,000	118,860,000	110,962,000
Service cost	9,373,000	7,101,000	6,140,000
Interest cost	7,570,000	7,195,000	6,596,000
Benefits paid	(5,760,000)	(6,522,000)	(6,589,000)
Actuarial (gain) loss	24,570,000	21,778,000	1,751,000
Net benefit obligation at end of year	184,165,000	148,412,000	118,860,000
Accumulated benefit obligation	150,800,000	124,700,000
Change in plan assets:
Fair value of plan assets at beginning of year	103,404,000	106,568,000	91,551,000
Actual return on plan assets	21,267,000	(6,642,000)	9,106,000
Employer contributions	15,000,000	10,000,000	12,500,000
Benefits paid	(5,760,000)	(6,522,000)	(6,589,000)
Fair value of plan assets at end of year	133,911,000	103,404,000	106,568,000
Funded status at end of year	(50,254,000)	(45,008,000)	(12,292,000)
Amounts recognized in the statement of financial position:
Noncurrent liabilities	(50,254,000)	(45,008,000)	(12,292,000)
Net amount recognized at end of year	(50,254,000)	(45,008,000)	(12,292,000)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Transition obligation	(32,000)	(37,000)	(42,000)
Prior service cost	(2,377,000)	(2,932,000)	(3,486,000)
Accumulated loss	(74,286,000)	(66,747,000)	(31,369,000)
Accumulated other comprehensive loss	(76,695,000)	(69,716,000)	(34,897,000)
Cumulative employer contributions in excess of net periodic benefit cost	26,441,000	24,708,000	22,605,000
Net amount recognized at end of year	(50,254,000)	(45,008,000)	(12,292,000)
Weighted average assumptions used to determine benefit obligation
Discount rate (as a percent)	4.22%	4.99%	6.00%
Rate of compensation increase (as a percent)	3.99%	4.04%	3.86%
Other Postretirement Benefits
Pension Plan and Postretirement Benefits Other than Pension
Age of employees after which the plan does not provide benefits	65 years
Change in projected benefit obligation:
Net benefit obligation at beginning of year	8,145,000	6,850,000	5,945,000
Service cost	693,000	558,000	443,000
Interest cost	400,000	402,000	364,000
Benefits paid	(560,000)	(554,000)	(528,000)
Actuarial (gain) loss	(223,000)	530,000	389,000
Retiree contributions	337,000	359,000	237,000
Net benefit obligation at end of year	8,792,000	8,145,000	6,850,000
Change in plan assets:
Employer contributions	223,000	195,000	291,000
Retiree contributions	337,000	359,000	237,000
Benefits paid	(560,000)	(554,000)	(528,000)
Funded status at end of year	(8,792,000)	(8,145,000)	(6,850,000)
Amounts recognized in the statement of financial position:
Current liabilities	(304,000)	(289,000)	(303,000)
Noncurrent liabilities	(8,488,000)	(7,856,000)	(6,547,000)
Net amount recognized at end of year	(8,792,000)	(8,145,000)	(6,850,000)
Amounts not yet reflected in net periodic benefit cost and included in accumulated other comprehensive income:
Prior service cost	(127,000)	(183,000)	(238,000)
Accumulated loss	(765,000)	(999,000)	(469,000)
Accumulated other comprehensive loss	(892,000)	(1,182,000)	(707,000)
Cumulative employer contributions in excess of net periodic benefit cost	(7,900,000)	(6,963,000)	(6,143,000)
Net amount recognized at end of year	$ (8,792,000)	$ (8,145,000)	$ (6,850,000)
Weighted average assumptions used to determine benefit obligation
Discount rate (as a percent)	4.18%	5.00%	6.00%
Term of forward rate implied by the Citigroup Pension Discount Curve	1 year