Long-Term Debt (Schedule of Long-Term Debt) (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt, gross	$ 776,500	$ 1,039,000
Less unamortized debt discount	413	437
Long-term debt	776,087	1,038,563
Credit facility \| Credit facility
Debt Instrument [Line Items]
Long-term debt, gross	222,500	485,000
Medium-term Notes \| 7.40% Medium-term Notes, Series A, due July 24, 2017
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,000	$ 2,000
Debt instrument, interest rate	7.40%	7.40%
Medium-term Notes \| 7.36% Medium-term Notes, Series A, due July 24, 2017
Debt Instrument [Line Items]
Long-term debt, gross	$ 15,000	$ 15,000
Debt instrument, interest rate	7.36%	7.36%
Medium-term Notes \| 7.23% Medium-term Notes, Series A, due July 28, 2017
Debt Instrument [Line Items]
Long-term debt, gross	$ 2,000	$ 2,000
Debt instrument, interest rate	7.23%	7.23%
Medium-term Notes \| 7.32% Medium-term Notes, Series A, due July 28, 2022
Debt Instrument [Line Items]
Long-term debt, gross	$ 20,000	$ 20,000
Debt instrument, interest rate	7.32%	7.32%
Medium-term Notes \| 7.60% Medium-term Notes, Series A, due July 26, 2027
Debt Instrument [Line Items]
Long-term debt, gross	$ 5,000	$ 5,000
Debt instrument, interest rate	7.60%	7.60%
Medium-term Notes \| 7.35% Medium-term Notes, Series A, due July 28, 2027
Debt Instrument [Line Items]
Long-term debt, gross	$ 10,000	$ 10,000
Debt instrument, interest rate	7.35%	7.35%
Medium-term Notes \| 7.125% Medium-term Notes, Series B, due February 15, 2028
Debt Instrument [Line Items]
Long-term debt, gross	$ 100,000	$ 100,000
Debt instrument, interest rate	7.125%	7.125%
Notes \| 4.625% Notes, due September 1, 2021
Debt Instrument [Line Items]
Long-term debt, gross	$ 400,000	$ 400,000
Debt instrument, interest rate	4.625%	4.625%

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20
-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 16
-Article 9

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Contractual interest rate for funds borrowed, under the debt agreement.

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&loc=d3e28878-108400

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