Debt (Senior Notes) (Details) (USD $)	3 Months Ended		0 Months Ended	3 Months Ended	12 Months Ended	0 Months Ended	3 Months Ended	12 Months Ended
	Mar. 31, 2012	Apr. 02, 2011	Jan. 17, 2006 6.125% Notes, Payable January 15, 2016 [Member]	Mar. 31, 2012 6.125% Notes, Payable January 15, 2016 [Member]	Dec. 31, 2009 6.125% Notes, Payable January 15, 2016 [Member]	Apr. 16, 2012 7.20% Senior Notes, Payable April 15, 2012 [Member]	Mar. 31, 2012 7.20% Senior Notes, Payable April 15, 2012 [Member]	Dec. 31, 2011 7.20% Senior Notes, Payable April 15, 2012 [Member]	Dec. 31, 2002 7.20% Senior Notes, Payable April 15, 2012 [Member]
Debt Instrument [Line Items]
Interest rate percentage				6.125%			7.20%	7.20%
Aggregate principal amount of debts			$ 900,000,000						$ 400,000,000
Remaining outstanding senior notes repaid								63,730,000
Increase in interest rate from rating agency downgrade				0.25%
Decrease in interest rate from rating agency upgrade				0.50%
Maximum increase in interest rate per agency from rating agency downgrade				1.00%
Increase in interest expense per quarter for .25% increase in notes per $100,000 of outstanding notes	22,498,000	26,595,000	63
Incremental increase in outstanding notes			100,000
Aggregate amount of increase in interest rate from rating agency downgrade					75
Aggregate amount of decrease in interest rate from rating agency downgrade				50
Repurchase price as percentage of principal amount of senior notes								102.72%
Company repaid principal amount						336,270,000	336,300,000
Debt instruments interest accrued						$ 12,106,000	$ 12,106,000
Notes payable, maturity date				Jan. 15, 2016			Apr. 15, 2012