Retirement Benefits (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Retirement Benefits [Abstract]
Number of unfunded non-qualified deferred compensation pension plans	2		2
Years of service credited under Supplemental Executive Retirement Plan (in years)	0.5Y		0.5Y
Defined benefit postretirement plans [Abstract]
Number of defined benefit postretirement plans	2		2
Number of defined postretirement plans providing medical benefits	1		1
Assumed submitted claims medical trend rate used to fix the amount of the Company's obligation (in hundredths)			9.25%
Year in which submitted claims medical trend rate was determined	1997		1997
Defined contribution retirement plans [Abstract]
Employer contribution match rate, U.S. employees (in hundredths)	75.00%		75.00%
Maximum employee contribution rate matched by employer, U.S. employees (in hundredths)	6.00%		6.00%
Maximum employee contribution rate over and above employee contribution rate matched by employer, U.S. employees (in hundredths)	18.00%		18.00%
Incremental rate by which employee contribution rate may be increased or decreased, U.S. employees (in hundredths)	1.00%		1.00%
Minimum employer contribution rate, Canadian employees (in hundredths)	3.00%		3.00%
Maximum employer contribution rate, Canadian employees (in hundredths)	8.00%		8.00%
Expense reocrded	$ 1,200,000	$ 1,000,000	$ 3,400,000	$ 2,800,000
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,723,000	1,414,000	5,168,000	4,242,000
Interest cost	2,959,000	2,896,000	8,878,000	8,689,000
Expected return on plan assets	(3,400,000)	(3,188,000)	(10,199,000)	(9,564,000)
Amortization of prior service cost	236,000	196,000	709,000	589,000
Amortization of unrecognized net loss (gain)	979,000	928,000	2,936,000	2,784,000
Net periodic benefit cost	2,497,000	2,246,000	7,492,000	6,740,000
Estimated future employer contributions for the next fiscal year	8,000,000		8,000,000		6,400,000
Employer contributions made during the period			200,000
Other Postretirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	32,000	23,000	96,000	69,000
Interest cost	87,000	88,000	260,000	263,000
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	13,000	13,000	38,000	38,000
Amortization of unrecognized net loss (gain)	(56,000)	(73,000)	(167,000)	(218,000)
Net periodic benefit cost	76,000	51,000	227,000	152,000
Estimated future employer contributions for the next fiscal year					500,000
Employer contributions made during the period			200,000
Estimated future employer contributions for the current fiscal year	$ 100,000		$ 100,000