Debt and Credit Facilities (Details) (USD $)	3 Months Ended		9 Months Ended													3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 5.4 percent senior notes due January 2015	Dec. 31, 2012 5.4 percent senior notes due January 2015	Sep. 30, 2013 8.4 percent senior notes due May 2017	Dec. 31, 2012 8.4 percent senior notes due May 2017	Sep. 30, 2013 6.6 percent senior notes due May 2020	Dec. 31, 2012 6.6 percent senior notes due May 2020	Sep. 30, 2013 5.4 percent senior notes due October 2022	Dec. 31, 2012 5.4 percent senior notes due October 2022	Sep. 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 June 2019	Sep. 30, 2013 0.25 percent convertible senior notes due June 2019	May 14, 2013 0.25 percent convertible senior notes due June 2019	Dec. 31, 2012 0.25 percent convertible senior notes due June 2019	Sep. 30, 2013 Seller-financed nonrecourse secured notes	Dec. 31, 2012 Seller-financed nonrecourse secured notes	Sep. 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 notes and convertible senior notes	$ 1,398,981,000		$ 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		$ 0
Debt discount	(2,527,000)		(2,527,000)		(3,000,000)
Senior notes and convertible senior notes, net	1,396,454,000		1,396,454,000		1,128,481,000
Secured notes payable	1,438,000		1,438,000		5,987,000															1,400,000	6,000,000
Total debt	1,397,892,000		1,397,892,000		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%		0.25%	0.25%		0.25%
Pretax charges related to early retirement of debt	0	9,146,000	0	9,146,000
Principal amount of senior notes issued																267,500,000
Conversion ratio of debt instrument																	0.0133
Debt instrument, initial conversion price (in dollars per share)																	$ 75.01
Debt instrument, conversion premium (as a percent)																	50.00%
Closing price of common stock (in dollars per share)																		$ 50.01
Ownership interest in guarantor subsidiaries (as a percent)			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 notice of redemption is provided																	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%
Redemption price as a percentage of principal amount of the notes																	100.00%
Amount of sinking fund																	0
Net proceeds from offering																260,100,000
Letters of credit outstanding	95,800,000		95,800,000		79,500,000
Maximum borrowing capacity																							$ 75,000,000	$ 50,000,000