Debt and Credit Facilities (Details) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Debt
Total senior notes	$ 1,131,481,000	$ 1,131,481,000
Debt discount	(2,846,000)	(3,000,000)
Senior notes, net	1,128,635,000	1,128,481,000
Secured notes payable	4,525,000	5,987,000
Total debt	1,133,160,000	1,134,468,000
Other debt disclosures
Letters of credit outstanding	80,500,000	79,500,000
5.4 percent senior notes due January 2015
Debt
Total senior notes	126,481,000	126,481,000
Other debt disclosures
Interest rate stated percentage	5.40%
8.4 percent senior notes due May 2017
Debt
Total senior notes	230,000,000	230,000,000
Other debt disclosures
Interest rate stated percentage	8.40%
1.6 percent convertible senior notes due May 2018
Debt
Total senior notes	225,000,000	225,000,000
Other debt disclosures
Interest rate stated percentage	1.60%
6.6 percent senior notes due May 2020
Debt
Total senior notes	300,000,000	300,000,000
Other debt disclosures
Interest rate stated percentage	6.60%
5.4 percent senior notes due October 2022
Debt
Total senior notes	250,000,000	250,000,000
Other debt disclosures
Interest rate stated percentage	5.40%
Seller-financed nonrecourse notes
Debt
Secured notes payable	4,500,000	6,000,000
Repurchase credit facility | JPMorgan Chase Bank, N.A. ("JPM")
Debt
Financial services credit facility	0	0
Other debt disclosures
Maximum borrowing capacity		$ 75,000,000	$ 50,000,000