6. Debt Maturity Summary (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 1,262,176	$ 1,215,000
Total Debt in Percentage	100.00%
Debt, Weighted Average Interest Rate	4.70%
2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 8,750
Percentage of Total Debt	0.70%
Debt, Weighted Average Interest Rate	2.30%
2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 102,500
Percentage of Total Debt	8.10%
Debt, Weighted Average Interest Rate	2.30%
2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 0
Percentage of Total Debt	0.00%
Debt, Weighted Average Interest Rate	0.00%
2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 50,926
Percentage of Total Debt	4.10%
Debt, Weighted Average Interest Rate	2.40%
2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 600,000
Percentage of Total Debt	47.50%
Debt, Weighted Average Interest Rate	5.90%
2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 250,000
Percentage of Total Debt	19.80%
Debt, Weighted Average Interest Rate	2.30%
2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 250,000
Percentage of Total Debt	19.80%
Debt, Weighted Average Interest Rate	5.60%
Unsecured Notes due 2021 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	5.875%
Debt Instrument, Maturity Date	Sep. 15, 2021
Unsecured Notes due 2023 [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	5.625%
Debt Instrument, Maturity Date	Jun. 15, 2023
Debt Instrument, Unamortized Discount	$ 1,700
AccThreeTermLoan [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Mar. 27, 2018
AccThreeTermLoan [Member] \| BeginningAprilOneTwoThousandSixteen [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 1,250
Debt Instrument, Date of First Required Payment	Apr. 01, 2016
AccThreeTermLoan [Member] \| BeginningAprilOneTwoThousandSeventeen [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 2,500
Debt Instrument, Date of Increased Required Payment	Apr. 01, 2017
Unsecured Term Loan [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jan. 21, 2022
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument, Maturity Date	Jul. 25, 2020
Fixed Rate Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Fixed Interest, Amount	$ 850,000	850,000
Percentage of Total Debt	67.00%
Long-term Debt, Percentage Bearing Fixed Interest, Percentage Rate	5.80%
Fixed Rate Debt [Member] \| 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 0
Fixed Rate Debt [Member] \| 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Fixed Rate Debt [Member] \| 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Fixed Rate Debt [Member] \| 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Fixed Rate Debt [Member] \| 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	600,000
Fixed Rate Debt [Member] \| 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Fixed Rate Debt [Member] \| 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	250,000
Floating Rate Debt [Member]
Debt Instrument [Line Items]
Long-term Debt, Percentage Bearing Variable Interest, Amount	$ 412,176	$ 365,000
Percentage of Total Debt	33.00%
Floating Rate Debt [Member] \| 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 8,750
Floating Rate Debt [Member] \| 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	102,500
Floating Rate Debt [Member] \| 2019 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Floating Rate Debt [Member] \| 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	50,926
Floating Rate Debt [Member] \| 2021 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	0
Floating Rate Debt [Member] \| 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	250,000
Floating Rate Debt [Member] \| 2023 [Member]
Debt Instrument [Line Items]
Debt Instrument, Periodic Payment, Principal	$ 0

Debt Instrument, Date of Increased Required Payment

No definition available.

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Period Type:	duration

- Definition

Percentage of Total Debt

+ References

No definition available.

+ Details

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Balance Type:	na
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- Definition

Total Debt in Percentage

+ References

No definition available.

+ Details

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- Definition

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.

+ References

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

+ Details

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

- Definition

Date the debt agreement requires the first payment to be made, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6479336&loc=d3e64711-112823

Reference 2: 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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of the required periodic payments applied to principal.

+ References

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, after accumulated amortization, of debt discount.

+ 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=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

+ Details

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

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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

- Definition

The interest rate applicable to the portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

+ References

+ Details

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Period Type:	instant

- Definition

The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a set, unchanging rate.

+ References

No definition available.

+ Details

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

- Definition

The portion of the carrying amount of long-term borrowings outstanding as of the balance sheet date, including current maturities, which accrues interest at a rate subject to change from time to time.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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- Details

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- Details

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- Details

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- Details

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