Debt and Credit Facilities (Details) (USD $)	3 Months Ended		6 Months Ended					1 Months Ended	3 Months Ended							3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 6.9 percent senior notes due June 2013	Dec. 31, 2011 6.9 percent senior notes due June 2013	Jul. 31, 2012 6.9 percent senior notes due June 2013 Subsequent Events	Jun. 30, 2011 5.4 percent senior notes due January 2015	Jun. 30, 2012 5.4 percent senior notes due January 2015	Dec. 31, 2011 5.4 percent senior notes due January 2015	Jun. 30, 2012 8.4 percent senior notes due May 2017	Dec. 31, 2011 8.4 percent senior notes due May 2017	Jun. 30, 2012 6.6 percent senior notes due May 2020	Dec. 31, 2011 6.6 percent senior notes due May 2020	Jun. 30, 2012 1.6 percent convertible senior notes due May 2018	Jun. 30, 2012 1.6 percent convertible senior notes due May 2018	May 10, 2012 1.6 percent convertible senior notes due May 2018	Jun. 30, 2012 Seller-financed nonrecourse notes	Dec. 31, 2011 Seller-financed nonrecourse notes	Jun. 30, 2012 Repurchase credit facility JPMorgan Chase Bank, N.A. ("JPM")	Dec. 31, 2011 Repurchase credit facility JPMorgan Chase Bank, N.A. ("JPM")
Debt
Total senior notes	$ 1,048,663,000		$ 1,048,663,000		$ 823,663,000	$ 167,182,000	$ 167,182,000			$ 126,481,000	$ 126,481,000	$ 230,000,000	$ 230,000,000	$ 300,000,000	$ 300,000,000	$ 225,000,000	$ 225,000,000
Debt discount	(3,339,000)		(3,339,000)		(3,647,000)
Senior notes, net	1,045,324,000		1,045,324,000		820,016,000
Secured notes payable	2,532,000		2,532,000		3,811,000														2,500,000	3,800,000
Total debt	1,047,856,000		1,047,856,000		823,827,000
Financial services credit facility	50,271,000		50,271,000		49,933,000																50,300,000	49,900,000
Other debt disclosures
Interest rate stated percentage						6.90%	6.90%	6.90%		5.40%	5.40%	8.40%	8.40%	6.60%	6.60%	1.60%	1.60%
Conversion ratio of debt instrument per $1,000 principal amount																	0.0312
Debt instrument, initial conversion price																$ 32.03	$ 32.03
Debt instrument, conversion premium																42.50%	42.50%
Closing price of common stock																		$ 22.48
Percentage ownership of Guarantor Subsidiaries			100.00%
Net proceeds from offering																218,800,000
Cash paid for redemption and repurchase of senior notes			0	28,222,000				177,200,000	28,200,000
Face value of debt instrument redeemed and repurchased								167,200,000	27,500,000
Loss on repurchase of debt	0	857,000	0	857,000				(9,100,000)	857,000
Letters of credit outstanding	37,900,000		37,900,000		66,000,000
Maximum borrowing capacity																					$ 60,000,000	$ 50,000,000