Long-Term Debt and Notes Payable - Long-Term Debt (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Gross amount	$ 1,239,000us-gaap_DebtInstrumentCarryingAmount	$ 1,039,000us-gaap_DebtInstrumentCarryingAmount
Less unamortized debt discount	431	437
Long-term debt	1,238,569us-gaap_LongTermDebtNoncurrent	1,038,563us-gaap_LongTermDebtNoncurrent
Credit facility
Debt Instrument [Line Items]
Gross amount	685,000	485,000
Medium-term Notes \| 7.40% Medium-term Notes, Series A, due July 24, 2017
Debt Instrument [Line Items]
Gross amount	2,000	2,000
Interest Rate	7.40%	7.40%
Medium-term Notes \| 7.36% Medium-term Notes, Series A, due July 24, 2017
Debt Instrument [Line Items]
Gross amount	15,000	15,000
Interest Rate	7.36%	7.36%
Medium-term Notes \| 7.23% Medium-term Notes, Series A, due July 28, 2017
Debt Instrument [Line Items]
Gross amount	2,000	2,000
Interest Rate	7.23%	7.23%
Medium-term Notes \| 7.32% Medium-term Notes, Series A, due July 28, 2022
Debt Instrument [Line Items]
Gross amount	20,000	20,000
Interest Rate	7.32%	7.32%
Medium-term Notes \| 7.60% Medium-term Notes, Series A, due July 26, 2027
Debt Instrument [Line Items]
Gross amount	5,000	5,000
Interest Rate	7.60%	7.60%
Medium-term Notes \| 7.35% Medium-term Notes, Series A, due July 28, 2027
Debt Instrument [Line Items]
Gross amount	10,000	10,000
Interest Rate	7.35%	7.35%
Medium-term Notes \| 7.125% Medium-term Notes, Series B, due February 15, 2028
Debt Instrument [Line Items]
Gross amount	100,000	100,000
Interest Rate	7.125%	7.125%
Notes Payable \| 4.625% Notes, due September 1, 2021
Debt Instrument [Line Items]
Gross amount	$ 400,000	$ 400,000
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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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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Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

+ Details

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Balance Type:	credit
Period Type:	instant

- Details

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Namespace Prefix:
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Balance Type:
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- Details

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- Details

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