Long-Term Debt - Schedule of Long-Term Debt (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Jan. 23, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Gross amount	$ 678,500		$ 554,000
Less amounts due within one year	17,000		24,000
Less unamortized debt discount	374		387
Less unamortized debt issuance costs	1,968		2,170
Total	659,158		527,443
Medium-term Notes \| 7.40% Medium-term Notes, Series A, due July 24, 2017
Debt Instrument [Line Items]
Gross amount	$ 0	$ 2,000	$ 2,000
Stated interest rate	7.40%	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
Stated 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
Stated 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
Stated 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	$ 0	$ 5,000	$ 5,000
Stated interest rate	7.60%	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
Stated 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
Stated 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
Stated interest rate	4.625%		4.625%

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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 FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Amount, after accumulated amortization, of debt discount.

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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=64930785&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=64930739&loc=d3e28878-108400

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