Debt and Credit Facilities (Details) (USD $)	12 Months Ended										12 Months Ended		12 Months Ended					12 Months Ended					12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2011 Senior notes	Dec. 31, 2013 5.4 percent senior notes due January 2015	Dec. 31, 2012 5.4 percent senior notes due January 2015	Dec. 31, 2013 8.4 percent senior notes due May 2017	Dec. 31, 2012 8.4 percent senior notes due May 2017	Dec. 31, 2013 6.6 percent senior notes due May 2020	Dec. 31, 2012 6.6 percent senior notes due May 2020	Dec. 31, 2012 5.4 percent senior notes due October 2022	Dec. 31, 2013 5.4 percent senior notes due October 2022	Dec. 31, 2012 6.9 percent senior notes due June 2013	Dec. 31, 2013 1.6 percent convertible senior notes due May 2018	Dec. 31, 2012 1.6 percent convertible senior notes due May 2018	May 10, 2012 1.6 percent convertible senior notes due May 2018	Dec. 31, 2011 1.6 percent convertible senior notes due May 2018	Dec. 31, 2013 1.6 percent convertible senior notes due May 2018 Maximum	Dec. 31, 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	Dec. 31, 2011 0.25 percent convertible senior notes due June 2019	Dec. 31, 2013 0.25 percent convertible senior notes due June 2019 Maximum	Dec. 31, 2013 Seller-financed nonrecourse secured notes	Dec. 31, 2012 Seller-financed nonrecourse secured notes	Dec. 31, 2013 Repurchase credit facility JPM	Dec. 31, 2012 Repurchase credit facility JPM	Dec. 31, 2011 Repurchase credit facility JPM
Debt
Total senior notes 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		$ 0
Debt discount	(2,362,000)	(3,000,000)
Senior notes and convertible senior notes, net	1,396,619,000	1,128,481,000
Secured notes payable	689,000	5,987,000																						689,000	6,000,000
Total debt	1,397,308,000	1,134,468,000
Financial services credit facility	73,084,000	0																								73,100,000	0
Maturities of debt
2014	73,760,000
2015	126,494,000
2016	0
2017	230,000,000
2018	225,000,000
After 2018	817,500,000
Total	1,472,754,000
Other debt disclosures
Interest rate stated percentage					5.40%		8.40%		6.60%		5.40%	5.40%	6.90%	1.60%	1.60%		1.60%		0.25%		0.25%	0.25%
Pretax charges related to early retirement of debt	0	9,146,000	1,608,000	1,600,000									9,100,000
Debt repurchased				51,500,000									167,200,000
Cash paid for redemption and repurchase	0	177,219,000	52,917,000	52,900,000									177,200,000
Principal amount of senior notes issued	267,500,000	475,000,000	0								250,000,000				225,000,000				267,500,000
Conversion ratio of debt instrument															0.0312168				0.0133307
Debt instrument, initial conversion price (in dollars per share)															$ 32.03				$ 75.01
Debt instrument, conversion premium (as a percent)															42.50%				50.00%
Closing price of common stock (in dollars per share)																$ 22.48				$ 50.01
Number of previous trading days considered for average closing price of common stock used to determine option price														10 days					10 days
Number of previous trading days considered for average closing price of common stock used to determine payment of tender offer														10 days					10 days
Regular cash dividend (in dollars per share)																		$ 0.03					$ 0.03
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				0
Net proceeds from offering											246,600,000				218,800,000				260,100,000
Cash proceeds of long-term debt	267,500,000	475,000,000	0
Letters of credit outstanding	93,600,000	79,500,000
Maximum borrowing capacity																											$ 75,000,000	$ 50,000,000