Debt Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2010
Debt Instrument [Line Items]
Promissory Notes	$ 72,900	$ 0
Total long-term debt	605,888	491,648
2012	0
2013	0
2014	200,000
2015	185,000
2016	72,900
Thereafter	150,000
Total	607,900
Letter of Credit [Member]
Debt Instrument [Line Items]
Credit Agreement	200,000	159,000
Senior Notes [Member] | 6.25% Interest Rate [Member]
Debt Instrument [Line Items]
Principal	185,000	185,000
Unamortized discount	(1,203)	(1,584)
Unamortized premium - designated fair value hedge	1,098	1,444
Notes Payable, Noncurrent	184,895	184,860
Senior Notes [Member] | 8.25% Interest Rate [Member]
Debt Instrument [Line Items]
Principal	150,000	150,000	150,000
Unamortized discount	(1,907)	(2,212)
Notes Payable, Noncurrent	$ 148,093	$ 147,788