Debt Instrument Tendered

No definition available.

Name:	nix_DebtInstrumentTendered
Namespace Prefix:	nix_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

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-Publisher FASB
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-Topic 835
-SubTopic 30
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+ Details

Name:	us-gaap_DebtInstrumentFaceAmount
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

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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+ Details

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

- 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

Fair value amount of debt instrument that was repurchased.

+ References

No definition available.

+ Details

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

- Definition

Face (par) amount of the original debt instrument that was repurchased.

+ References

No definition available.

+ Details

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

- Definition

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
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+ Details

Name:	us-gaap_GainsLossesOnExtinguishmentOfDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

The cash inflow from additional borrowings, net of cash paid to third parties in connection with debt origination.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3255-108585

+ Details

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

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_SeniorNotesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nix_SeniorNotesRedeemed2018Note1Member
Namespace Prefix:
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Balance Type:
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- Details

Name:	us-gaap_DebtInstrumentAxis=nix_A6.80NotesDue2019MemberMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nix_A6.125NotesDue2022Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nix_A3.65NotesDue2023Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nix_A5.45Notesdue2020Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=nix_SeniorNoteRedeemed2018Note2Member
Namespace Prefix:
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Long-Term Debt (Narrative) (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended	9 Months Ended
	Jul. 16, 2018	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Jun. 11, 2018	Mar. 15, 2018
Debt Instrument [Line Items]
Loss on early extinguishment of long-term debt		$ (33.0)	$ 0.0	$ 12.5	$ (45.5)	$ (111.5)
Debt Instrument Tendered					$ 209.0
Senior Notes
Debt Instrument [Line Items]
Debt Instrument, Face Amount							$ 350.0
Senior Note Redeemed 2018 Note 1 \| Senior Notes
Debt Instrument [Line Items]
Debt Instrument, Repurchase Amount								$ 275.1
Debt Instrument, Interest Rate, Stated Percentage								6.40%
6.80% Notes Due 2019
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.80%			6.80%
6.125% Notes Due 2022
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		6.125%			6.125%
3.65% Notes Due 2023
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		3.65%			3.65%
3.65% Notes Due 2023 \| Senior Notes
Debt Instrument [Line Items]
Proceeds from Debt, Net of Issuance Costs					$ 346.6
5.45% Notes due 2020
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		5.45%			5.45%
Senior Note Redeemed 2018 Note 2
Debt Instrument [Line Items]
Loss on early extinguishment of long-term debt	$ 33.0
Debt Instrument, Repurchased Face Amount	$ 551.1