Long-Term Debt (Narrative) (Details) (USD $)	12 Months Ended				12 Months Ended				12 Months Ended			12 Months Ended			12 Months Ended				12 Months Ended				12 Months Ended			12 Months Ended		12 Months Ended				12 Months Ended		12 Months Ended			12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Multi-Currency Line Of Credit [Member]	Dec. 31, 2012 Domestic And Multi-Currency Line Of Credit Up To $100,000 Due 2013 [Member]	Dec. 31, 2012 Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member]	Dec. 31, 2011 Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member]	Dec. 31, 2010 Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member]	Dec. 31, 2012 Variable Rate Senior Unsecured Note Due 2015 [Member]	Dec. 31, 2011 Variable Rate Senior Unsecured Note Due 2015 [Member]	Dec. 31, 2010 Variable Rate Senior Unsecured Note Due 2015 [Member]	Dec. 31, 2012 Credit Agreement [Member]	Nov. 29, 2011 Credit Agreement [Member]	Mar. 30, 2011 Credit Agreement [Member]	Dec. 31, 2012 5.25% Convertible Senior Unsecured Notes Due 2029 [Member]	Dec. 31, 2011 5.25% Convertible Senior Unsecured Notes Due 2029 [Member]	Dec. 31, 2010 5.25% Convertible Senior Unsecured Notes Due 2029 [Member]	May 19, 2009 5.25% Convertible Senior Unsecured Notes Due 2029 [Member]	Dec. 31, 2012 Letter Of Credit Facility [Member]	Mar. 30, 2011 Letter Of Credit Facility [Member]	Dec. 31, 2012 Standby Letters Of Credit [Member]	Nov. 29, 2011 Additional Credit Facility [Member]	Dec. 31, 2012 Domestic And Multi Currency Line And The 2015 Variable Rate Notes [Member]	Dec. 31, 2012 Series A and B Notes [Member]	Aug. 28, 2012 Series A and B Notes [Member]	Dec. 31, 2012 Series A Notes [Member]	Aug. 28, 2012 Series A Notes [Member]	Dec. 31, 2012 Series B Notes [Member]	Aug. 28, 2012 Series B Notes [Member]	Dec. 31, 2012 LIBOR [Member] Variable Rate Senior Unsecured Note Due 2015 [Member]	Dec. 31, 2012 Agent Base Rate [Member] Variable Rate Senior Unsecured Note Due 2015 [Member]	Dec. 31, 2012 Minimum [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member] D	Dec. 31, 2011 Minimum [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member] D	Dec. 31, 2012 Minimum [Member] Series A and B Notes [Member]	Dec. 31, 2012 Minimum [Member] LIBOR [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member]	Dec. 31, 2012 Minimum [Member] Agent Base Rate [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member]	Dec. 31, 2012 Maximum [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member] D	Dec. 31, 2011 Maximum [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member] D	Dec. 31, 2012 Maximum [Member] LIBOR [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member]	Dec. 31, 2012 Maximum [Member] Agent Base Rate [Member] Domestic And Multi-Currency Line Of Credit Up To $280,000 Due 2015 [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity				$ 50,000,000	$ 100,000,000	$ 280,000,000						$ 380,000,000	$ 430,000,000	$ 330,000,000								$ 100,000,000
Debt instrument maturity, year									2015						2029
Debt Instrument, Face Amount									50,000,000						1,000										52,000,000		47,000,000		5,000,000
Debt instrument basis spread on variable rate																														3.50%	2.00%				2.00%	0.50%			3.25%	1.75%
Weighted average interest rate						3.06%	3.08%		3.75%	3.81%
Commitment fee percentage						0.38%																										0.25%					0.50%
Debt Instrument, Periodic Payment, Principal									2,100,000																	9,400,000		7,000,000						1,000,000
Number Of Pricing Tranches						3
Letter of credit facility, amount																				20,000,000	17,700,000
Debt instrument, maturity date					May 29, 2013				Mar. 31, 2015						May 15, 2029											Aug. 28, 2019		Aug. 28, 2022
Debt instrument maturity days																																2	3				31	31
Debt Instrument, Issuance Date												Mar. 30, 2011			May 19, 2009				Mar. 30, 2011					Aug. 28, 2012
Debt Instrument, Date of First Required Payment									Mar. 31, 2012																	Aug. 28, 2015		Aug. 28, 2016
Debt facility issuance costs															3,900,000								2,600,000
Aggregate principal amount of convertible senior notes																		115,000,000
Debt instrument, interest rate															5.25%											6.00%		6.58%
Conversion rate of notes (in shares)															39.2157
Conversion price per share															$ 25.50
Repurchase price as percentage on principal amount															100.00%
Long Term Debt	578,330,000	537,291,000							41,667,000	50,000,000					110,197,000	107,058,000
Unamortized discount															4,800,000	7,900,000
Interest expenses															9,200,000	8,900,000	8,700,000
Amortization of debt discount	3,811,000	3,566,000	3,340,000												3,200,000	2,900,000	2,700,000
Contractual interest expenses															6,000,000	6,000,000	6,000,000
Effective interest rate															8.46%	8.46%
Convertible notes, if-converted value in excess of principal amount															59,700,000
Carrying amount of equity component recorded as additional paid-in capital															$ 9,400,000	$ 9,400,000
Numbers Of Notes Series																								2
Debt Instrument, Frequency of Periodic Payment									Quarterly															Annually