Long-Term Debt and Notes Payable - Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified																							9 Months Ended
	Sep. 30, 2014	Dec. 31, 2013	Sep. 30, 2014 Credit facility	Dec. 31, 2013 Credit facility	Sep. 30, 2014 Medium-term Notes 7.40% Medium-term Notes, Series A, due July 24, 2017	Dec. 31, 2013 Medium-term Notes 7.40% Medium-term Notes, Series A, due July 24, 2017	Sep. 30, 2014 Medium-term Notes 7.36% Medium-term Notes, Series A, due July 24, 2017	Dec. 31, 2013 Medium-term Notes 7.36% Medium-term Notes, Series A, due July 24, 2017	Sep. 30, 2014 Medium-term Notes 7.23% Medium-term Notes, Series A, due July 28, 2017	Dec. 31, 2013 Medium-term Notes 7.23% Medium-term Notes, Series A, due July 28, 2017	Sep. 30, 2014 Medium-term Notes 7.32% Medium-term Notes, Series A, due July 28, 2022	Dec. 31, 2013 Medium-term Notes 7.32% Medium-term Notes, Series A, due July 28, 2022	Sep. 30, 2014 Medium-term Notes 7.60% Medium-term Notes, Series A, due July 26, 2027	Dec. 31, 2013 Medium-term Notes 7.60% Medium-term Notes, Series A, due July 26, 2027	Sep. 30, 2014 Medium-term Notes 7.35% Medium-term Notes, Series A, due July 28, 2027	Dec. 31, 2013 Medium-term Notes 7.35% Medium-term Notes, Series A, due July 28, 2027	Sep. 30, 2014 Medium-term Notes 7.125% Medium-term Notes, Series B, due February 15, 2028	Dec. 31, 2013 Medium-term Notes 7.125% Medium-term Notes, Series B, due February 15, 2028	Sep. 30, 2014 Notes Payable 4.625% Notes, due September 1, 2021	Dec. 31, 2013 Notes Payable 4.625% Notes, due September 1, 2021	Sep. 30, 2014 Senior Term Loans Senior Term Loans, (floating rate interest LIBOR plus 1.625%)	Dec. 31, 2013 Senior Term Loans Senior Term Loans, (floating rate interest LIBOR plus 1.625%)	Sep. 30, 2014 London Interbank Offered Rate (LIBOR) Senior Term Loans Senior Term Loans, (floating rate interest LIBOR plus 1.625%)
Debt Instrument [Line Items]
Gross amount	$ 632,900	$ 1,154,000	$ 78,900	$ 0	$ 2,000	$ 2,000	$ 15,000	$ 15,000	$ 2,000	$ 2,000	$ 20,000	$ 20,000	$ 5,000	$ 5,000	$ 10,000	$ 10,000	$ 100,000	$ 100,000	$ 400,000	$ 400,000	$ 0	$ 600,000
Less amounts due within one year	0	60,000
Less unamortized debt discount	442	459
Long-term debt	$ 632,458	$ 1,093,541
Interest Rate					7.40%		7.36%		7.23%		7.32%		7.60%		7.35%		7.125%		4.625%
Floating rate interest spread																							1.625%