Debt and Credit Facilities (Details) (USD $)	6 Months Ended										3 Months Ended			3 Months Ended	6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 5.4 percent senior notes due January 2015	Dec. 31, 2012 5.4 percent senior notes due January 2015	Jun. 30, 2013 8.4 percent senior notes due May 2017	Dec. 31, 2012 8.4 percent senior notes due May 2017	Jun. 30, 2013 6.6 percent senior notes due May 2020	Dec. 31, 2012 6.6 percent senior notes due May 2020	Jun. 30, 2013 5.4 percent senior notes due October 2022	Dec. 31, 2012 5.4 percent senior notes due October 2022	Jun. 30, 2012 1.6 percent convertible senior notes due May 2018	Jun. 30, 2013 1.6 percent convertible senior notes due May 2018	Dec. 31, 2012 1.6 percent convertible senior notes due May 2018	Jun. 30, 2013 0.25 percent convertible senior notes due 2019	Jun. 30, 2013 0.25 percent convertible senior notes due 2019	May 14, 2013 0.25 percent convertible senior notes due 2019	Jun. 30, 2013 Seller-financed nonrecourse notes	Dec. 31, 2012 Seller-financed nonrecourse notes	Jun. 30, 2013 Repurchase credit facility JPMorgan Chase Bank, N.A. ("JPM")	Dec. 31, 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 and convertible senior notes	$ 1,398,981,000	$ 1,131,481,000	$ 126,481,000	$ 126,481,000	$ 230,000,000	$ 230,000,000	$ 300,000,000	$ 300,000,000	$ 250,000,000	$ 250,000,000		$ 225,000,000	$ 225,000,000	$ 267,500,000	$ 267,500,000
Debt discount	(2,688,000)	(3,000,000)
Senior and convertible senior notes, net	1,396,293,000	1,128,481,000
Secured notes payable	3,792,000	5,987,000															3,800,000	6,000,000
Total debt	1,400,085,000	1,134,468,000
Financial services credit facility																			0	0
Other debt disclosures
Interest rate stated percentage			5.40%		8.40%		6.60%		5.40%		1.60%	1.60%		0.25%	0.25%
Principal amount of senior notes issued											225,000,000			267,500,000
Conversion ratio of debt instrument per $1,000 principal amount														0.0133
Debt instrument, initial conversion price (in dollars per share)														$ 75.01	$ 75.01
Debt instrument, conversion premium (as a percent)														50.00%	50.00%
Closing price of common stock (in dollars per share)																$ 50.01
Ownership interest in guarantor subsidiaries (as a percent)	100.00%														100.00%
Minimum number of days within 30 consecutive trading days in which the closing price of the entity's common stock must exceed the conversion price for the notes to be redeemed for cash														20 days
Number of consecutive trading days during which the closing price of the entity's common stock must exceed the conversion price for at least 20 days in order for the notes to be redeemed for cash														30 days
Number of trading days preceding the date on which it provides notice of redemption														5 days
Percentage of the conversion price that the closing sales price of the entity's common stock must exceed in order for the notes to be redeemed for cash														130.00%	130.00%
Redemption price as percentage of principal amount of the notes														100.00%	100.00%
Amount of Sinking fund														0	0
Net proceeds from offering											218,800,000			260,100,000
Letters of credit outstanding	87,500,000	79,500,000
Maximum borrowing capacity																				$ 75,000,000	$ 50,000,000